By the One who took his soul, meaning the Prophet ﷺ, he did not pass away until most of his prayers were performed sitting, except for the obligatory ones. The deed most beloved to him was the one a servant maintained consistently, even if it was small.
حدثنا عبد الرزاق قال حدثنا سفيان عن أبي إسحاق عن أبي سلمة بن عبد الرحمن
والذي توفى نفسه تعني النبي ﷺ ما توفي حتى كانت أكثر صلاته قاعدا إلا المكتوبة وكان أعجب العمل إليه الذي يدوم عليه العبد وإن كان يسيرا
Its chain of transmission is sahih. Nasai reported it in 3/222, no. 1653 and following, in the chapter on Qiyam al-Layl, and Ibn Majah in 1/387, no. 1225, in the chapter on prayer, and in 2/1416, no. 4237, in the chapter on asceticism.