Abu Dawud Book 2, Hadith Number 1347.
Narated By ‘Aisha, Ummul Mu’minin : Sa’d ibn Hisham said: I came to Medina and called upon ‘Aisha, and said to her: Tell me about the prayer of the Apostle of Allah (PBUH).
She said: The Apostle of Allah (PBUH) used to lead the people in the night prayer, and then go to his bed and sleep. When midnight came he got up, went to answer the call of nature and to perform ablution with water. Having performed ablution, he entered the mosque and prayed eight rak’ahs.
To my mind he performed the recitation of the Qur’an, bowing and prostrating equally. He then observed witr with one rak’ah and prayed two rak’ahs sitting. Then he lay down on the ground. Sometimes Bilal came to him and called him for prayer. He then dozed, and sometimes I doubted whether he dozed or not, till he (Bilal) called him for prayer.
This is the prayer he offered till he grew old or put on weight. She then mentioned how he put on weight as Allah wished.