The Messenger of Allah ﷺ used to pray witr with nine rakahs, but when he grew old and his body became heavy, he prayed witr with seven.
حدثنا محمد بن فضيل عن الأعمش عن عمارة بن عمير عن يحيى بن الجزار
كان رسول الله ﷺ يوتر بتسع فلما أسن وثقل أوتر بسبع
Its isnad is sahih. Yahya al-Jazzar is al-Urani, deemed trustworthy, and his hadith appears in Muslim and the Four (Sunan compilers). This hadith has already appeared in similar wording at 22214.