Musnad Ahmad 1156:211

Rawh narrated to us, Ibn Jurayj narrated to us, he said: Yahya ibn Abdullah ibn Muhammad ibn Sayfi informed me that Ikrimah ibn Abd al-Rahman informed him that Umm Salamah informed him that the Prophet, peace and blessings of Allah be upon him, swore that he would not enter upon some of his family for a month. When twenty-nine days had passed, he went to them in the morning or in the evening. It was said to him: "You swore, O Prophet of Allah, that you would not enter upon them for a month?" He said: "Indeed, the month is twenty-nine days."