Musnad Ahmad 1153:2104

'Abd al-Samad and Abu 'Amir—the meaning is the same—said: Hisham narrated to us, from Yahya, from Abu Salamah, who said: 'Aishah said: "The Messenger of Allah (may Allah bless him and grant him peace) would not fast more in a year than he fasted in Sha'ban, for he used to fast all of Sha'ban." And he used to say: "Take upon yourselves of the deeds what you can bear, for Allah does not grow weary until you grow weary." And indeed, "the most beloved of deeds to the Messenger of Allah (may Allah bless him and grant him peace) were those which were performed consistently, even if they were few. And if he performed a prayer, he would be consistent in it."