Musnad Ahmad 1156:76

Muhammad ibn Bakr and ‘Abd al-Razzaq narrated to us, they said: Ibn Jurayj narrated to us, he said: ‘Abdullah ibn Abi Mulaykah informed me—‘Abd al-Razzaq said: ‘Abdullah ibn Abi Mulaykah—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 Prophet’s prayer at night. She said: "He would pray the final ‘Isha’, then he would perform Tasbih (voluntary prayer), then he would pray after that whatever Allah willed of the night. Then he would finish and sleep for a duration similar to the time he had prayed. Then he would wake up from that sleep of his, and he would pray for a duration similar to the time he had slept, and his final prayer would be until the morning."