Sunan of Abu Dawood Book 5, Hadith 1343

حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، بِإِسْنَادِهِ نَحْوَهُ قَالَ ‏:‏ يُصَلِّي ثَمَانِ رَكَعَاتٍ لاَ يَجْلِسُ فِيهِنَّ إِلاَّ عِنْدَ الثَّامِنَةِ، فَيَجْلِسُ فَيَذْكُرُ اللَّهَ عَزَّ وَجَلَّ، ثُمَّ يَدْعُو، ثُمَّ يُسَلِّمُ تَسْلِيمًا يُسْمِعُنَا، ثُمَّ يُصَلِّي رَكْعَتَيْنِ وَهُوَ جَالِسٌ بَعْدَ مَا يُسَلِّمُ، ثُمَّ يُصَلِّي رَكْعَةً، فَتِلْكَ إِحْدَى عَشْرَةَ رَكْعَةً يَا بُنَىَّ، فَلَمَّا أَسَنَّ رَسُولُ اللَّهِ صلى الله عليه وسلم وَأَخَذَ اللَّحْمَ أَوْتَرَ بِسَبْعٍ، وَصَلَّى رَكْعَتَيْنِ وَهُوَ جَالِسٌ بَعْدَ مَا يُسَلِّمُ، بِمَعْنَاهُ إِلَى مُشَافَهَةً ‏.‏

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 the eight rak'ahs. He would sit, make mention of Allah, supplicate Him and then utter the salutation so loudly that we could hear it. He would then pray two rak'ahs sitting after he had uttered the salutation. Then he would pray one rak'ah, and that made eleven rak'ahs, O my son. When the Messenger of Allah (ﷺ) grew old and became fleshy, he offered seven rak'ahs of witr, and then he would pray two rak'ahs sitting after he had uttered the salutation. The narrator narrated the tradition to the same effect till the end