Musnad Abu Ya'la al-Mawsili 209:95

Abu Khaythama narrated to us, Abd al-Rahman narrated to us, Shu'bah narrated to us, from Abu Ishaq, who said: I heard Abu Salamah narrating from Umm Salamah, who said: The Messenger of Allah (may Allah bless him and grant him peace) did not die until most of his prayer was while he was sitting. And the most beloved deed to Allah, the Almighty and Majestic, was that which the servant persisted in, even if it were small.