Mukhtasar Bukhari

Narrated from Umm al-Mu'mineen Umm Salamah (may Allah be pleased with her) that once the Prophet Muhammad (peace be upon him) took an oath of abstention from his pure wives for a month (i.e., swore to abstain from relations). Then, when twenty-nine days had passed, in the morning or after midday, the Prophet (peace be upon him) went to his wives. Someone said to him, "You had sworn that you would not go to your wives for a month." The Prophet (peace be upon him) replied, "A month can also be of twenty-nine days."