Musnad Ahmad 1153:1653

Yahya narrated to us, he said: Sufyan narrated to us, he said: Ash'ath narrated to me, from his father, from Masruq, from Aisha, she said: The Messenger of Allah, peace and blessings of Allah be upon him, "used to like the consistent deed." He said: So I said: At what time of the night would he stand [for prayer]? She said: "When he heard the caller."