Musnad Ahmad 1153:527

Abu al-Mughirah narrated to us, he said: al-Awza'i narrated to us, he said: Yahya ibn Abi Kathir narrated to us, from Abu Salamah, from Aisha, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Take upon yourselves of deeds that which you are able, for Allah, the Mighty and Majestic, does not grow weary until you grow weary." Aisha said: "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." Aisha said: "And the Prophet, peace and blessings of Allah be upon him, when he performed a prayer, he would persist in it." Abu Salamah said: "Allah, the Mighty and Majestic, said: {Those who are persistent in their prayer} [Al-Ma'arij: 23]."