From Sahih Bukhari (Hadith 6461):
Narrated Masruq:I asked `Aisha "What deed was the most beloved to the Prophet?" She said, "The regular constant one." I said, "At what time did he use to get up at night (for the Tahajjud night prayer)?' She said, "He used to get up on hearing (the crowing of) the cock (the last third of the night)
Reference: Bukhari 6461