Musnad Ahmad 830:148

Bahz narrated to us, Hammam narrated to us, Qatadah informed us, from al-Hasan, from Abu Bakrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not one of you say, 'I stood [in prayer] the whole of Ramadan.'" Qatadah said: And Allah knows best, he feared self-praise for his nation, or he said: "There must be one who sleeps or one who is heedless."