Musnad Ahmad 1153:2058

Rawh narrated to us, Hisham bin Abi Abdullah narrated to us, from Yahya bin Abi Kathir, from Abu Salamah, that Aisha narrated to him that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Take upon yourselves of deeds that which you can endure, for Allah, the Almighty and Majestic, does not grow weary until you grow weary." And the most beloved of prayer to the Messenger of Allah, peace and blessings of Allah be upon him, was that which one persists in, even if it be little. And when he performed a prayer, he would persist in it.