Musnad Ahmad 830:116

Yazid narrated to us, Hammam informed us; and 'Affan narrated to us, Hammam narrated to us, Qatadah informed us, 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.'" Qatadah said: "And Allah knows best, he feared for his nation self-praise (tazkiyah)." 'Affan said: Or he said: "There must be one who sleeps or one who is heedless."