Musnad Abu Ya'la al-Mawsili 150:29

Ishaq narrated to us, al-Nadr narrated to us, Shu'bah narrated to us, from Sa'd, who said: I heard Abu Salamah from 'Aishah, who said: The Messenger of Allah (may Allah bless him and grant him peace) was asked: Which deed is most beloved to Allah? He said: The most constant of them, even if it is little.