The Prophet ﷺ used to pray nine rakats during the night. When he grew heavy with age, he prayed seven.
حدثنا عبد الرزاق قال أخبرنا الثوري عن الأعمش عن عمارة عن يحيى بن الجزار
كان النبي ﷺ يصلي من الليل تسعا فلما ثقل وأسن صلى سبعا
Its isnad is sahih. It was previously mentioned in 24570. Umarah is Ibn Umayr, and Yahya bin Jazzar is al-Arni, and both are reliable, their hadith is in Muslim.