Musnad Ahmad 30:1479

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abd al-Rahman al-A'raj narrated to us, he said: I heard Abu Hurairah saying: The Messenger of Allah, may Allah bless him and grant him peace, said: "Take upon yourselves of deeds that which you can endure, for the best of deeds is that which is most constant, even if it be little."