Musnad Ahmad 175:32

'Attab ibn Ziyad narrated to us, he said: 'Abdullah informed us, he said: Ibn Lahi'ah informed us, he said: Musa ibn Jubayr, the freed slave of Banu Salamah, narrated to me that he heard 'Abdullah ibn Ka'b ibn Malik narrating from his father, who said: The people, during Ramadan, when a man fasted and reached the evening and slept, food, drink, and women became forbidden to him until he broke his fast the next day. 'Umar ibn al-Khattab returned from the Prophet, may Allah bless him and grant him peace, one night, having stayed up with him, and he found his wife had slept. He desired her, but she said: "I have already slept." He said: "You have not slept," then he had intercourse with her. Ka'b ibn Malik did the same. 'Umar went early to the Prophet, may Allah bless him and grant him peace, and informed him. Then Allah, the Exalted, revealed: "Allah knew that you used to deceive yourselves, so He accepted your repentance and pardoned you." [Al-Baqarah: 187]