Musnad Ahmad 830:44

Yazid narrated to us, Hammam informed us, from Qatadah, from al-Hasan, from Abu Bakrah, who said: 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,' nor 'I fasted the whole of it.'" Al-Hasan said: And Yazid said once: Qatadah said: "Allah knows best, I fear for his nation [the implication of] self-purification, or is it inevitable that there be one who sleeps or one who is heedless?"