The Prophet ﷺ swore that he would not visit some of his wives for a month. When twenty-nine days had passed, he went to them in the morning or the evening. He was asked, "Prophet of Allah, did you not swear that you would not visit them for a month?"He replied, "A month can be twenty-nine days."
أن النبي ﷺ حلف لا يدخل على بعض أهله شهرا، فلما مضى تسعة وعشرون يوما غدا عليهن أو راح، فقيل له: يا نبي الله، حلفت أن لا تدخل عليهن شهرا؟ قال: إن الشهر يكون تسعة وعشرين يوما.
Bukhari reported it in the Book of Marriage (5202), and Muslim in the Book of Fasting (25:1085), via Ibn Jurayj, who said, "Yahya bin Abdullah bin Sayfi informed me, that Ikrima bin Abdul-Rahman bin Harith informed him, that Umm Salama informed him," and he mentioned it.