Musnad Ahmad 1156:153

'Abd al-Razzaq narrated to us, Ibn Jurayj narrated to us, he said: 'Abd Allah ibn Abi Mulaykah said: Ya'la ibn Mamlak informed me that he asked Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, about the prayer of the Prophet, may Allah bless him and grant him peace, at night. She said: "The Prophet, may Allah bless him and grant him peace, used to pray the last 'Isha', then he would perform tasbih (voluntary prayer), then he would pray after it whatever Allah, the Mighty and Majestic, willed of the night. Then he would depart and sleep for as long as he had prayed, then he would wake up from that sleep of his, and he would pray for as long as he had slept, and that last prayer of his would be until the morning."