Musnad Ahmad 1153:1125

Muhammad ibn Ja'far and Rawh narrated to us, they said: Shu'bah narrated to us, from al-Ash'ath ibn Sulaym, from his father. And Rawh said: He said: Al-Ash'ath ibn Sulaym informed me that he heard his father narrating from Masruq, who said: I asked 'Aishah: Which deed was most beloved to the Messenger of Allah, may Allah bless him and grant him peace? She said: "The constant one." Ibn Ja'far said: So I said: And at what time would he stand [for prayer]? She said: "When he heard the caller."