Musnad Ahmad 830:115

Muhammad ibn Ja‘far narrated to us, Sa‘id and ‘Abd al-Wahhab narrated to us, from Sa‘id, from Qatadah, from al-Hasan, from Abu Bakrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Let not one of you say, 'I stood [in prayer] the whole of Ramadan.'" He said: "And Allah knows best, he feared for his nation that they might purify themselves [i.e., claim purity for themselves]." ‘Abd al-Wahhab said: "And Allah knows best, did he fear purification for his nation? Or did he say: 'There must be sleep or heedlessness'?"