Musnad Ahmad 30:1749

Ibrahim narrated to us, Ibn al-Mubarak narrated to us, from Kathir ibn Zayd, 'Amr ibn Tamim narrated to me, from his father, that he heard Abu Hurairah 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. No month has passed upon the believers that is better for them than it, and no month has passed upon the hypocrites that is worse for them than it. Indeed, Allah (Mighty and Majestic is He) writes its reward and its supererogatory acts before it enters, and He writes its burden and its misery before it enters. That is because the believer prepares in it strength for worship through spending, while the hypocrite prepares to follow the heedlessness of the people and to follow their faults. So it is a bounty for the believer, which the wicked person seizes."