It was narrated from Umm Salamah that:
the Messenger of Allah (ﷺ) swore to keep away from some of his wives for a month. On the twenty-ninth day, in the evening or the morning, it was said: "O Messenger of Allah, only twenty-nine days have passed." He said: "The month is twenty-nine days." (IT)
English reference (IT): Vol. 3, Book 10, Hadith 2061 (IT)
Arabic reference (IT): Book 10, Hadith 2139 (IT)