Musnad Ahmad 1153:1455

'Abd al-Rahman narrated to us, he said: Shu'bah narrated to us, from Sa'd ibn Ibrahim, from Abu Salamah, from 'Aishah, she said: The Messenger of Allah, peace and blessings of Allah be upon him, was asked: Which deeds are most beloved to Allah the Almighty? He said: "The most constant of them, even if it is little." He said: And I heard him—meaning Abu Salamah—narrating from 'Aishah, or from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, he said: "Take upon yourselves of deeds that which you are able to bear."