Why did ‘Umar (may Allah be pleased with him) call uniting the people in praying Taraweeh behind one imam an innovation (bid‘ah)?
Question

‘Umar ibn al-Khattaab (may Allah be pleased with him) united the people in praying Tarweeh, and said: That is a good innovation (bid‘ah hasanah). Why did he describe it as an innovation, even though he was one of the four Rightly Guided Caliphs whose words and deeds we are instructed to follow, as they are part of the Sunnah?
Praise be to Allah.Firstly: 
Al-Bukhaari (2010) narrated from ‘Abd ar-Rahmaan ibn ‘Abd al-Qaari’ that he said: I went out with ‘Umar ibn al-Khattaab (may Allah be pleased with him) one night in Ramadan to the mosque, where we saw the people in scattered groups, one man praying by himself, and another man praying with a group of people following his prayer. ‘Umar said: I think that if I unite these people behind one reciter, it will be better. Then he decided to do that, so he united them behind Ubayy ibn Ka‘b. Then I went out with him on another night, and the people were all praying behind their reciter. ‘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. 
With regard to ‘Umar’s words “what a good innovation (bid‘ah) this is”, what he meant by calling it an innovation was in the linguistic sense, i.e., it was something new that the people had never done before. 
That was because gathering the people in Ramadan every night behind one imam on a continual and organised basis had not been done before. Therefore the context indicates that what was meant was the linguistic meaning, and he did not mean it in the technical shar‘i sense, which means introducing something into the religion that is not part of it, whilst attributing it to the religion. Taraweeh prayer is part of the religion and is something that is prescribed and encouraged; similarly, offering this prayer in congregation is also encouraged and recommended, and the basis for that is proven in the actions and words of the Prophet (blessings and peace of Allah be upon him). 
The scholars have stated that: 
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 
This is a description in linguistic terms, not in a technical shar‘i sense. That is because the word innovation (bid‘ah), in linguistic terms, includes everything that is introduced without precedent.
With regard to innovation (bid‘ah) in the technical, shar‘i sense, this refers to something for which there is no shar‘i evidence. 
If the words of the Messenger of Allah (blessings and peace of Allah be upon him) indicate that something is recommended or obligatory after his death, or is indicated in general terms, even though it was not done until after his death – such as the book of charity [a document that listed the rates of zakaah on livestock] that Abu Bakr (may Allah be pleased with him) issued – then doing that action after his death may correctly be called an innovation in linguistic terms, because it was something new. 
In fact the very religion that the Prophet (blessings and peace of Allah be upon him) brought may be called an innovation and may be called something new in linguistic terms, as the envoys of Quraysh said to the Negus (ruler of Abyssinia) concerning the companions of the Prophet (blessings and peace of Allah be upon him) who migrated to Abyssinia: These people left the religion of their forefathers and did not enter the religion of the King [i.e., Christianity]; rather they brought an innovated religion that is not known. 
Moreover, a deed which is supported by the Qur’an and Sunnah is not an innovation in shar‘i terms, even if it may be described as an innovation in linguistic terms. The word innovation is more general in meaning in the linguistic sense than in the technical shar‘i sense. 
It is known that the words of the Prophet (blessings and peace of Allah be upon him), “Every innovation is a going astray,” do not refer to every new thing that is done. In fact the religion of Islam – and indeed every religion brought by the Messengers – was a new thing. Rather what he meant is what is newly introduced of deeds (in the religion) that were not prescribed by him (blessings and peace of Allah be upon him). 
As that is the case, they used to pray the qiyaam of Ramadan (i.e., Taraweeh) at the time of the Prophet (blessings and peace of Allah be upon him) both in congregation and individually. He said to them on the third or fourth night, when they gathered: “Nothing prevented me from coming out to join you except the fact that I would not like it to be made obligatory for you. So pray in your houses, for the best prayer a man can offer is in his house, except the prescribed obligatory prayers.” 
So he (blessings and peace of Allah be upon him) explained the reason why he did not come out (to lead them in Taraweeh prayers), which was the fear that it might be made obligatory. Thus it is known that the reason for his coming out to join them was still valid, and that were it not for the fear of it being made obligatory, he would have come out to join them. 
But at the time of ‘Umar’s caliphate, he (may Allah be pleased with him) united them behind a single reciter and put lamps in the mosque. This way of doing it – which was to gather in the mosque behind a single imam, with lamps in the mosque – was something that they had not done before, hence it was called an innovation, because it may be described as such in linguistic terms, but it was not an innovation in shar‘i terms, because the Sunnah indicates that it was a righteous deed, were it not for the fear that it might be made obligatory. Fear that it might be made obligatory ceased with the death of the Prophet (blessings and peace of Allah be upon him), so there was no longer any reason not to do it. 
End quote from Iqtida’ as-Siraat al-Mustaqeem (2/95-97) 
Ibn Rajab (may Allah have mercy on him) said: 
With regard to what was said by some of the earlier generations about regarding some innovations as good, that refers to innovation in the linguistic sense, not in the technical shar‘i sense. An example of that is what ‘Umar (may Allah be pleased with him) said, when he united the people in praying qiyaam in Ramadan behind a single imam in the mosque; when he came out and saw the people praying in that manner, he said: What a good innovation this is. And it was narrated from him that he said: If this is an innovation, then what a good innovation. What he meant was that this deed had not been done in this manner before this time, but it had a basis in Islamic teaching that may be referred to, such as the fact that the Prophet (blessings and peace of Allah be upon him) used to encourage and urge people to pray qiyaam in Ramadan. At his time, the people used to pray qiyaam in the mosque, in scattered groups and individually, and he (blessings and peace of Allah be upon him) led his companions in praying qiyaam in Ramadan for several nights. Then he stopped doing that, on the basis that he feared that it might be made obligatory for them, then they would be unable to do it, but there was no fear of that after he (blessings and peace of Allah be upon him) died. 
End quote from Jaami‘ al-‘Uloom wa’l-Hikam (2/783) 
Shaykh Ibn Baaz (may Allah have mercy on him) said: 
The scholars have explained that what ‘Umar meant by that was the linguistic sense, according to Arabic usage, because he (may Allah be pleased with him) united the people behind a single imam, whereas at the time of the Prophet (blessings and peace of Allah be upon him) and Abu Bakr as-Siddeeq they used to pray in scattered groups. During his time (may Allah be pleased with him), he united them behind a single imam; he passed by them one night as they were praying and said: What a good innovation this is, i.e., uniting them behind a single imam in an ongoing and organised fashion. 
This has to do with the linguistic meaning according to Arabic usage; he did not mean that it was an innovations in the technical, shar‘i sense. It is not possible that he (may Allah be pleased with him) could have introduced or approved of (reprehensible) innovations. 
End quote from Fataawa Noor ‘ala ad-Darb (3/33) 
Shaykh al-Albaani (may Allah have mercy on him) said: 
The words of ‘Umar, “What a good innovation this is”, do not refer to innovation in the technical shar‘i sense, which means introducing something into the religion with no precedent. Rather he was referring to innovation in a linguistic sense, which is something new that was not known before he introduced it. There can be no doubt that offering Taraweeh prayer in congregation behind a single imam was not known or practised during the caliphate of Abu Bakr and the first half of ‘Umar’s caliphate, so in that sense it was something new. But because it was in accordance with what the Prophet (blessings and peace of Allah be upon him) did, it is Sunnah and is not an innovation, and he only described it is good because of that. 
End quote from Salaat at-Taraweeh (p. 50) 
For more information, please see question no. 183220 
And Allah knows best.