Musnad Ahmad 1156:133

Husayn ibn Muhammad narrated to us, he said: Isra'il narrated to us, from Abu Ishaq, from Abu Salamah, from Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, she said: "By the One who took his soul, the Prophet, may Allah bless him and grant him peace, did not die until most of his prayer was while sitting, except for the prescribed prayer. And the most beloved of deeds to him was that which the servant persists in, even if it were little."