From Sahih Bukhari (Hadith 4508):
Narrated Al-Bara':When the order of compulsory fasting of Ramadan was revealed, the people did not have sexual relations with their wives for the whole month of Ramadan, but some men cheated themselves (by violating that restriction). So Allah revealed: "Allah is aware that you were deceiving yourselves but He accepted your repentance and forgave you
Reference: Bukhari 4508