Musnad Ahmad 1153:2371

Hassan bin Musa and Husayn bin Muhammad narrated to us, they said: Shayban narrated to us, from al-Ash’ath, from his father, from Masruq, who said: I said to Aisha, "Which deed was most pleasing to the Messenger of Allah, may Allah bless him and grant him peace?" She said, "He used to love that which is continuous." I said, "At what time did he used to pray?" She said, "When he heard the caller (the adhan), he would stand and pray."