Abu Dawud Book 2, Hadith Number 1338.
Chapter : On the number of rakahs of the prayer at night.
Narated By N/A : The above mentioned tradition has also been narrated by Qatadah through a different chain of narrators. This version adds: He (the Prophet) used to pray eight rak’ahs during which he did not sit except after eighth rak’ah. He would sit, make a mention of Allah, supplicate Him and then utter the salutations so loudly that we could hear it. He would then pray two rak’ahs sitting after he had uttered salutation. Then he would pray one rak’ah, and that made eleven rak’ahs, sonny. When the Apostle of Allah (PBUH) grew old and became fleshy, he offered seven rak’ahs of witr, and then prayed two rak’ahs sitting after he had uttered the salutation. The narrator narrated the tradition to the same effect till the end.