Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The month of Ramadan has cast its shade over you." The Messenger of Allah (peace and blessings be upon him) said on oath: "No month has passed over the Muslims that is better for them than it, nor any month over the hypocrites that is worse for them than it." The Messenger of Allah (peace and blessings be upon him) stated on oath: "Allah Almighty writes the reward of its voluntary deeds before it even begins, and He writes its sins and its wretchedness before it begins as well. That is because the believer prepares for worship in it, while the hypocrite prepares for the heedlessness of the Muslims and pursues their faults. So it is a gain for the believer, which the wicked one seizes for himself."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8502