Sahih Bukhari | Volume 7 | Hadith Number 131
Narrated by Ibn 'Abbas
One morning we saw the wives of the Prophet weeping, and everyone of them had her family with her, I went to the mosque and found that it was crowded with people. Then 'Umar bin Al-Khattab came and went up to the Prophet who was in his upper room. He greeted him but nobody answered. He greeted again, but nobody answered. Then the gatekeeper called him and he entered upon the Prophet, and asked, "Have you divorced your wives?" The Prophet, said, "No, but I have taken an oath not to go to them for one month." So the Prophet stayed away (from his wives) for twenty nine days and then entered upon them.