Abu Dawud Book 2, Hadith Number 1372.

Chapter : Night prayers during ramadhan.

Narated By Abu Hurairah : The Apostle of Allah (PBUH) came out and saw that the people were praying during (the night) of Ramadhan in the corner of the mosque. He asked: Who are those people? It was said to him that those were the people who had not learnt the Qur’an. But Ubayy b. K’ab is praying and they would pray behind him. The Prophet (PBUH) said: They did right and it is good what they did.

Abu Dawud said: This tradition is not strong; the narrator Muslim b. Khalid is weak.