Ibn Majah: Hadith 2063

"The Messenger of Allah (ﷺ) swore that he would not enter upon his wives for a month, and he stayed for twenty-nine days until, on the eve of the thirtieth, he entered upon me. I said: 'You swore not to enter upon us for a month.' He said: 'The month may be like this,' and he held up his (ten) fingers three times; 'or the month may be like this,' and he held up his fingers three times, keeping one finger down on the third time."