Musnad Abu Ya'la al-Mawsili 209:55

Muhammad ibn Bashar narrated to us, Muhammad—meaning Ghundar—narrated to us, Shu'bah narrated to us, he said: I heard Abu Ishaq, that he heard Abu Salamah narrating from Umm Salamah, that she said: The Messenger of Allah (may Allah bless him and grant him peace) did not die until most of his prayer was while sitting, other than the obligatory prayer. And the most beloved of deeds to him was that which was most constant, even if it were little.