From Sahih Muslim (Hadith 2520):
Zuhri reported that (once) the Messenger of Allah (ﷺ) took an oath that he would not go to his wives for one Month. Zuhri said that 'Urwa narrated to him from 'A'isha (Allah be pleased with her) that she said:When twenty-nine nights were over, which I had counted, the Messenger of Allah (ﷺ) came to me (he came to me first of all). I said: Messenger of Allah, you had taken an oath that you would not come to us for a month, whereas you have come after twenty nine days which I have counted. Whereupon he said: The month may also consist of twenty-nine days
Reference: Muslim 2520