Musnad Ahmad 30:3652

'Abd al-Malik ibn 'Amr narrated to us, Kathir ibn Zayd narrated to us, 'Amr ibn Tamim narrated to me, my father informed me that he heard Abu Hurayrah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "This month of yours has overshadowed you. By the oath of the Messenger of Allah (peace and blessings of Allah be upon him), no month has ever passed for the Muslims that is better for them than it, and no month has ever passed for the hypocrites that is worse for them than it. By the oath of the Messenger of Allah (peace and blessings of Allah be upon him), Allah writes its reward and its voluntary acts, and He writes its burden and its misery, before it even enters. That is because the believer prepares in it the strength of expenditure for worship, while the hypocrite prepares in it the seeking of the heedlessness of the believers and their faults. So it is a bounty for the believer, which the wicked person seizes."