Musnad Ahmad 1153:611

Bahz narrated to us, he said: Shu'bah narrated to us, from al-Ash'ath ibn Sulaym, from his father, from Masruq, who said: I said to 'Aishah: "Which deed was most beloved to the Messenger of Allah, may Allah bless him and grant him peace?" She said: "The consistent one." I said: "And at what hour would he rise?" She said: "When he heard the caller."