It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) swore that he would not go in to some of his wives for a month. When twenty-nine days had passed, he (peace and blessings be upon him) went to them in the morning or the evening. It was said: O Messenger of Allah, did you not swear that you would not go in to them for a month? He (peace and blessings be upon him) said: "A month can also be twenty-nine days."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19987