Maliks Muwatta Book 7, Hadith Number 23.
Section : The Command to Pray the Witr.
Yahya related to me from Malik from Abd al Karim ibn Abi’l-Mukhariq al-Basri from Said ibn Jubayr that Abdullah ibn Abbas slept, and when he woke up, he said to his servant, “Go and see what the people have done,” (by that time his sight had gone.) The servant went out and returned saying, “The people have left from subh,” so Abdullah ibn Abbas got up and prayed witr and then prayed subh.