When the opening of this Surah was revealed, the Companions stood praying (most of the night) until their fett swelled, and the concluding verses were not revealed for twelve months from heaven. At last the concluding verses were revealed and the prayer at night became voluntary after it was obligatory. I said: Tell me about the witr of the Prophet (ﷺ). She replied: He used to pray eight rak'ahs, sitting only during the eighth of them. Then he would stand up and pray another rak'ahs. He would sit only after the eighth and the ninth rak'ahs. He would utter salutation only after the ninth rak'ah. He would then pray two rak'ahs sitting and that made eleven rak'ahs, O my son. But when he grew old and became fleshy he observed a witr of seven, sitting only in sixth and seventh rak'ahs, and would utter salutation only after the seventh rak'ah. He would then pray two rak'ahs sitting, and that made nine rak'ahs, O my son. The Messenger of Allah (ﷺ) would not pray through a whole night, or recite the whole Qur'an in a night or fast a complete month except in Ramadan. When he offered prayer, he would do that regularly. When he was overtaken by sleep at night, he would pray twelve rak'ahs.
The narrator said: I came to Ibn 'Abbas and narrated all this to him. By Allah, this is really a tradition. Has I been on speaking terms with her, I would have come to her and heard it from her mouth. I said: If I knew that you were not on speaking terms with her, I would have never narrated it to you.
In-book : Book 5, Hadith 93
English translation : Book 5, Hadith 1337