Abu Dawud Book 2, Hadith Number 1424.
Chapter : Supplications during the witr.
Narated By Ubayy ibn Ka’b : Al-Hasan reported: Umar ibn al-Khattab gathered the people (in tarawih prayer) behind Ubayy ibn Ka’b (who led them). He used to lead them for twenty days (during Ramadan, and would not recite the supplication except in the second half of it (i.e. Ramadan). When the last ten days remained, he kept away from them, and prayed in his house. They used to say: Ubayy ran away.
Abu Dawud said: This tradition shows that whatever has been reported about the recitation of the supplication is not tenable. Moreover, these two traditions from Ubayy b. K’ab indicate that another tradition which tells that the Prophet (PBUH) recited the supplication is weak.