Musnad Ahmad 1153:1968

Yazid narrated to us, he said: Hisham informed us, from al-Hasan, from Sa’d ibn Hisham, who said: I arrived in Medina and entered upon ‘Aishah. She said to me: "Who are you?" I said: "Sa’d ibn Hisham ibn ‘Amir." She said: "May Allah have mercy on your father." I said: "Inform me about the prayer of the Messenger of Allah, peace and blessings of Allah be upon him." She said: "Did the Messenger of Allah, peace and blessings of Allah be upon him, not recite?" I said: "Yes, but inform me." She said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray the final Isha prayer with the people, then he would retire to his bed. When the middle of the night arrived, he would rise to his purification, perform ablution, then enter the mosque and pray eight rak'ahs, making the recitation, bowing, and prostration equal in them. Then he would perform witr with one rak'ah, then pray two rak'ahs while sitting. Then he would lay his head down, and perhaps Bilal would come and notify him of the prayer before he dozed off—and perhaps I am in doubt whether he dozed off or did not doze off—until he notified him of the prayer." She said: "That was the prayer of the Messenger of Allah, peace and blessings of Allah be upon him, until he grew old and gained weight. He would pray the Isha with the people, then retire to his bed. When the middle of the night arrived, he would rise to his purification, perform ablution, then enter the mosque and pray six rak'ahs, making the recitation, bowing, and prostration equal in them. Then he would perform witr with one rak'ah, then pray two rak'ahs while sitting. And perhaps he did not doze off until Bilal came and notified him of the prayer—and perhaps I am in doubt whether he dozed off or did not doze off."