Recommended Recitation In the Two Sunnah Rak‘ahs Of Fajr and Maghrib
Question

Is reciting Surat al-Kafirun and Surat al-Ikhlas at the beginning and end of the day – that is, in the two Sunnah rak‘ahs of Fajr and Maghrib –recommended (mustahabb) and a Sunnah that was narrated from the Messenger of Allah (blessings and peace of Allah be upon him)?
Praise be to Allah.What to recite in Sunnahs of Fajr and Maghrib?
Yes, it is recommended to recite Surat al-Kafirun and Surat al-Ikhlas in the two Sunnah rak‘ahs of Fajr and Maghrib. This is proven in the sahih Sunnah of the Messenger of Allah (blessings and peace of Allah be upon him).
Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (blessings and peace of Allah be upon him) recited in the two Sunnah rak‘ahs of Fajr {Qul ya ayyuha’l-kafirun (Say, "O disbelievers)} [al-Kafirun 109] and {Qul huwa Allahu ahad (Say, He is Allah, [who is] One)} [al-Ikhlas 112].
Ibn ‘Umar (may Allah be pleased with him) said: I watched the Messenger of Allah (blessings and peace of Allah be upon him) twenty times reciting in the two rak‘ahs after Maghrib and in the two rak‘ahs before Fajr, {Qul ya ayyuha’l-kafirun (Say, "O disbelievers)} [al-Kafirun 109] and {Qul huwa Allahu ahad (Say, He is Allah, [who is] One)} [al-Ikhlas 112]. (Narrated by al-Nasai, 992)
An-Nawawi said in al-Majmu‘ (3/385): 
“Its isnad is jayyid. It was classed as sahih by al-Albani in as-Silsilah as-Sahihah (3328) and by Shaykh Ahmad Shakir in Tahqiq al-Musnad (8/89).”
Al-Mubarakfuri said in Tuhfat al-Ahwadhi (2/418):
“That is, in the first rak‘ah of each of them he would recite {Qul ya ayyuha’l-kafirun (Say, O disbelievers)} [al-Kafirun 109] and in the second {Qul huwa Allahu ahad (Say, He is Allah , [who is] One)} [al-Ikhlas 112].”
The scholars have stated that it is recommended to recite these two surahs in the Sunnah prayers of Fajr and Maghrib, in accordance with these hadiths. (See: al-Mughni (1/435); Mughni al-Muhtaj (1/464); al-Fatawa al-Fiqhiyyah al-Kubra (1/192); al-Mawsu‘ah al-Fiqhiyyah (27/159)
Why should we recite Surat al-Kafirun and Surat al-Ikhlas in Fajr and Maghrib Sunnah prayers?
With regard to the wisdom behind reciting these two surahs, that is because they include all three types of Tawhid . The surah {Qul huwa Allahu ahad (Say, He is Allah, [who is] One)} [al-Ikhlas 112] refers to tawhid ar-rububiyyah (oneness of divine Lordship ) and tawhid al-asma wa’s-sifat (oneness of the divine names and attributes ), for it affirms that Allah, may He be exalted, is one God and denies that He has any son, father or peer, and that in addition to that, He is as-Samad, in Whom are combined all attributes of perfection.
And the surah {Qul ya ayyuhu’l-kafirun (Say, O disbelievers)} [al-Kafirun 109] refers to tawhid al-‘ibadah (oneness of worship), which means that one should not worship anyone or anything except Allah, and should not associate anyone with Him in worship. Hence the Messenger (blessings and peace of Allah be upon him) used to start the day with them in the Sunnah prayer of Fajr , and he would end the day with them in the Sunnah prayer of Maghrib. In as-Sunan, it is narrated that he used to recite these surahs in Witr, so they were also a conclusion to the deeds of the night, just as they were a conclusion to the deeds of the day.” (This was stated by Ibn al-Qayyim in Badai‘ al-Fawaid, 1/145-146)
And Allah knows best.