Musnad Ahmad 30:1249

Abu Bakr al-Hanafi narrated to us, he said: Kathir ibn Zayd narrated to us, from ‘Amr ibn Tamim, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "By the oath of the Messenger of Allah, no month has come upon the Muslims that is better for them than Ramadan, and no month has come upon the hypocrites that is worse for them than Ramadan. That is because of what the believers prepare in it of strength for worship, and what the hypocrites prepare in it of people's heedlessness and their faults. It is the spoils of the believer which the wicked one seizes."