The Messenger of Allah ﷺ said, "The month is twenty-nine days," and he clapped his hands twice, then clapped a third time while folding in his thumb.Ayesha said, "May Allah forgive Abu Abdul-Rahman. He is mistaken. The Messenger of Allah ﷺ had withdrawn from his wives for a month, then came down after twenty-nine days. They said, 'Messenger of Allah, you have come down after twenty-nine days.' He replied, 'The month can be twenty-nine days.'"
حدثنا يزيد أخبرنا محمد عن يحيى بن عبد الرحمن بن حاطب قال
قال رسول الله ﷺ الشهر تسع وعشرون وصفق بيديه مرتين ثم صفق الثالثة وقبض إبهامه فقالت عائشة غفر الله لأبي عبد الرحمن إنه وهل إنما هجر رسول الله ﷺ نساءه شهرا فنزل لتسع وعشرين فقالوا يا رسول الله إنك نزلت لتسع وعشرين فقال إن الشهر يكون تسعا وعشرين
Its isnad is sahih. The hadith from this route was mentioned by Hafiz in Fath al-Bari 4:109, and he also attributed it to Ibn Abu Shayba.This objection from Ayesha is forced, for Ibn Umar did not mean that the month is always twenty-nine days, and this is not understood from his statement. Rather, he means exactly what she said and reported, that the month can be twenty-nine days.Bukhari (4:108-109) and Muslim (1:298-299) reported through the route of Saeed bin Amr, from Ibn Umar, from the Prophet ﷺ, that he said, "We are an unlettered nation, we do not write and we do not calculate. The month is like this and like this," meaning sometimes twenty-nine and sometimes thirty. The wording is Bukhari's. It will also come again in the Musnad at 5017.See also what has already passed in the Musnad of Ibn Umar at 4488, 4611, 4815, in the Musnad of Umar at 222, in the Musnad of Ibn Abbas at 2103, 3158, and in the Musnad of Ibn Masud at 3776, 3871, 4209, 4300. Ayesha reported something similar to what Ibn Masud reported, as will come later (6:90h).