Narrated Masruq (Radi-Allahu 'anhu):
I asked 'Aisha "What deed was the most beloved to the Prophet? (Sallallahu
'Alaihi Wa Sallam)" 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)."
Bukhari Vol. 8 : No. 468