Musnad Ahmad 1156:55

Yahya ibn Ishaq narrated to us, he said: Layth ibn Sa’d informed me, he said: Abdullah ibn Abi Mulaykah narrated to us, from Ya’la ibn Mamlak, he said: I asked Umm Salamah about the prayer of the Messenger of Allah, peace and blessings of Allah be upon him, at night and his recitation. She said: "What do you have to do with his prayer and his recitation? He used to pray for as long as he slept, and he used to sleep for as long as he prayed." And she described his recitation as being distinct, letter by letter.