The du‘aa’ (supplication) that the Sahaabah used to recite in Qunut al-Witr in the second half of Ramadan during the caliphate of ‘Umar ibn al-Khattaab
Question

Which dua did the sahaba used to recite in the midle of ramadan during the time, when Umar bin al-khattab was khalif?
Praise be to Allah.Firstly:
There are a number of hadiths which speak of Qunut in Witr, the most famous of which is that which was narrated by Imam Ahmad in al-Musnad (1718), Abu Dawood in his Sunan (1425), at-Tirmidhi in his Sunan (464), an-Nasaa’i in his Sunan (1745) and Ibn Maajah in his Sunan (1178) from al-Hasan ibn ‘Ali (may Allah be pleased with him), who said: The Messenger of Allah (blessings and peace of Allah be upon him) taught me words to say in Qunut al-Witr:
“Allaahumma ihdini feeman hadayta wa ‘aafini feeman ‘aafayta wa tawallani feeman tawallayta wa baarik li feema a‘tayta, wa qini sharra ma qadayta, fa innaka taqdi wa la yuqda ‘alayk, innahu laa yadhillu man waalayta, tabaarakta Rabbana wa ta‘aalayta
(O Allah, guide me among those whom You have guided, pardon me among those whom You have pardoned, turn to me in friendship among those to whom You have turned in friendship, and bless me in what You have bestowed, and save me from the evil of what You have decreed. For verily You decree and none can turn back Your decree; and he is not humiliated whom You have befriended. Blessed are You, O Lord, and Exalted).”
This hadith is saheeh; it was classed as saheeh by an-Nawawi in Khulaasat al-Ahkaam (1/455); Ibn Hajar in Muwaafaqat al-Khabari al-Khabara (1/333); Ibn al-Mulaqqin in al-Badr al-Muneer (3/630); and Shaykh al-Albaani in Saheeh Sunan Abi Dawood (1281).
Secondly:
With regard to the du‘aa’ that the Sahaabah regularly recited in Qunut al-Witr during the caliphate of Ameer al-Mu’mineen ‘Umar ibn al-Khattaab, it was narrated by Ibn Khuzaymah in his Saheeh (1100) via ‘Urwah ibn az-Zubayr that ‘Abd ar-Rahmaan ibn ‘Abd al-Qaari – who during the caliphate of ‘Umar ibn al-Khattaab was in charge of the bayt al-maal (treasury) along with ‘Abdullah ibn al-Arqam – narrated that ‘Umar went out one night in Ramadan, and ‘Abd ar-Rahmaan ibn ‘Abd al-Qaari went out with him. He walked around the mosque, and the people in the mosque were scattered, one man praying by himself, and one man praying with a group following him in prayer. ‘Umar said: by Allah, I think that if we bring them together behind one reciter (imam), that will be better.
Then ‘Umar decided to go ahead and do that, and he instructed Ubayy ibn Ka‘b to lead them in praying qiyaam in Ramadan.
Then ‘Umar came out and saw the people praying behind their reciter (imam), and ‘Umar said: What a good innovation this is, but what they sleep and miss is better than what they are doing – referring to prayer at the end of the night, whereas the people were praying qiyaam at the beginning of the night.
The people used to pray qiyaam at the beginning of the night, and they used to curse the disbelievers in the middle of the night:
“Allahumma qaatil al-kafarata alladheena yasuddoona ‘an sabeelika wa yukadhdhiboona rusulaka wa laa yu’minoona bi wa‘dika wa khaalif bayna kalimatihim wa alqi fi quloobihim ar-ru‘ba wa alqi ‘alayhim rijzaka wa ‘adhaabaka, ilaah al-haqq 
(O Allah, destroy the disbelievers who bar people from Your path, disbelieve Your messengers and do not believe in Your promise; create disunity among them, cast fear into their hearts and send Your punishment upon them, O God of truth).”
Then he would send blessings upon the Prophet (blessings and peace of Allah be upon him), and offer supplication for the Muslims, asking for whatever he could of good, then he would pray for forgiveness for the believers.
He (the narrator) said: And when he had finished cursing the disbelievers, sending blessings upon the Prophet, praying for forgiveness for the believing men and women, and asking for their needs, he would say:
“Allahumma iyyaaka na‘budu wa laka nusalli wa nasjud, wa ilayka nas‘a wa nahfid wa narju rahmataka rabbana, wa nakhaafu ‘adhaabak al-jadd, inna ‘adhaabaka liman ‘aadayta mulhaq
(O Allah, You alone we worship, and to You alone we pray and prostrate, and for Your sake we strive. We hope for Your mercy, our Lord, and we fear Your stern punishment, for Your punishment is inevitable for those whom You take as enemies).”
Then he would say takbeer and prostrate.
Its isnad is saheeh. It was classed as saheeh by Shaykh al-Albaani (may Allah have mercy on him) in Qiyaam Ramadaan (p. 31).
A similar report was also narrated from Mu‘aadh ibn al-Haarith Abu Haleemah al-Ansaari, who is known as Mu‘aadh al-Qaari’. He was one of the Sahaabah who were well versed in the Qur’an, as he was the reciter and imam of the Ansaar. He was killed in the battle of al-Harrah in 63 AH. Ibn Hajar said in al-Isaabah (6/110): He is the one whom ‘Umar appointed to lead Taraaweeh prayer in Ramadan. End quote.
It is proven that he used to offer a similar supplication to this when praying qiyaam in Ramadaan. It was narrated from him by Abu Dawood in Masaa’il al-Imam Ahmad (p. 96) via Muhammad ibn Sireen who said: Among the supplications that Mu‘aadh al-Qaari’ used to offer in qiyaam – that is, night prayers in Ramadan – was:
“Allahumma ‘adhdhib al-kafarat alladheena yasuddoona ‘an sabeelika wa yukadhdhiboona rusulak. Allahumma alqi fi quloobihim ar-ru‘ba wa khaalif bayna kalimihim, wa anzil ‘alayhim rijzaka wa ‘adhaabaka, wa zidhum ru‘ban ‘ala ru‘bihim. Allahumma qaatil kafarata ahl il-kitaab illadheena yasuddoona ‘an sabeelika wa yukadhdhiboona rusulaka. Wa Allahumma alqi fi quloobihim ar-ru‘ba wa Allahumma khaalif bayna kalimihim wa anzil ‘alayhim rijzaka wa ‘adhaabaka, wa zidhum ru‘ban ‘ala ru‘bihim. Allahumma aghfir li’l-mu’mineena wa’l-mu’minaat, wa’l-Muslimeena wa’l-Muslimaat, wa aslih dhaata baynihim wa allif bayna quloobihim waj‘al quloobahum ‘ala quloobi akhyaarihim wa azwi‘hum an yashkuru ni‘mataka allati an‘amta ‘alayhim, wa an yoofu bi ‘ahdika allahi ‘aahadtahum ‘alayhi wa ansurhum ‘ala ‘aduwwika wa ‘aduwwihim, ilaah al-haqq.
(O Allah, punish the disbelievers who bar people from Your path and disbelieve Your Messengers. O Allah, cast fear into their hearts, create disunity among them, send Your punishment upon them, and add fear to their fear. O Allah, destroy the disbelievers among the People of the Book who bar people from Your path and disbelieve in Your Messengers. O Allah, cast fear into their hearts. O Allah, create disunity among them, send Your punishment upon them, add fear to their fear. O Allah, forgive the believing men and women, the Muslim men and women, reconcile between them, create harmony among them, and make their hearts like the hearts of the best of them. Enable them to give thanks for Your blessing that you have bestowed upon them, and to be faithful to Your covenant that You have made with them; and support them against Your enemies and their enemies, O God of truth.)”
It was narrated from ‘Umar himself that he used to say in the du‘aa’ of Qunut:
“Allahumm aghfir lana wa li’l-mu’mineena wa’l-mu’minaati wa’l-muslimeena wa’l-muslimaat, wa allif bayna quloobihim wa aslih dhaata baynihim, wa’nsurhum ‘ala ‘aduwwika wa ‘aduwwihim. Allahumm al‘an kafarata ahl il-kitaab illadheena yasuddoona ‘an sabeelika wa yukadhdhiboona rusulaka wa yuqaatiloona awliya’aka. Allahumma khaalif bayna kalimatihim wa zalzil aqdaamahum wa anzil bihim ba’sak alladhi laa tarudduhu ‘an il-qawm il-mujrimeen.
Bismillah ir-rahmaan ir-raheem, Allahumma inna nasta‘eenuka wa nastaghfiruka wa nuthni ‘alayka wa la nakfuruka wa nakhla‘u wa natruku man yafjuruka.
Bismillah ir-rahmaan ir-raheem, Allahumma iyyaaka na‘budu wa laka nusalli wa nasjud, wa laka nas‘a wa nahfid, nakhsha ‘adhaabak al-jadda wa narju rahmataka, inna ‘adhaabaka bi’l-kaafireena mulhaq
(O Allah, forgive us, and the believing men and women, and the Muslim men and women.  Create harmony between them and reconcile between them, and support them against Your enemies and their enemies. O Allah, curse the disbelievers among the People of the Book who bar people from Your path and disbelieve in Your Messengers, and who fight Your close friends. O Allah, create division among them, shake the earth beneath their foot, and send down upon them Your punishment from which You do not spare the people who are evildoers.
In the name of Allah, the Most Gracious, the Most Merciful: O Allah, we seek Your help and Your forgiveness; we praise You and are not ungrateful to You, and we shun and reject those who rebel against You.
In the name of Allah, the Most Gracious, the Most Merciful: O Allah, You alone we worship, and to You alone we pray and prostrate, and for Your sake we strive. We fear Your stern punishment and we hope for Your mercy, our Lord, for Your punishment is inevitable for the disbelievers).”
This du‘aa’ was narrated by ‘Abd ar-Razzaaq in his Musannaf (4969); Ibn Abi Shaybah in his Musannaf (7027); and al-Bayhaqi in as-Sunan al-Kubra (2/210). This report was classed as saheeh by al-Bayhaqi following his narration of it, and also by Ibn al-Mulaqqin in al-Badr al-Muneer (4/371).
If the worshippers combine all these reports that were narrated from the Prophet (blessings and peace of Allah be upon him) in Qunut al-Witr, and what was narrated from his companions (may Allah be pleased with them), this is a good thing, and there is nothing in that  which is too long or too difficult for people.
If they vary between these supplications, sometimes reciting one and sometimes reciting another, that is also good.  All these options are good, if Allah wills.
For more information, please see the answers to questions no. 14093 and 81083 .
And Allah knows best.