Musnad Abu Ya'la al-Mawsili 150:69

Muhammad ibn Abdullah ibn Numayr narrated to us, Muhammad ibn Fudayl narrated to us, from al-A’mash, from Abu Salih, who said: Aisha and Umm Salama were asked: "Which deed was most beloved to the Messenger of Allah, may Allah bless him and grant him peace?" They both said: "That which its doer persists upon, even if it is little."