Musnad Ahmad 27:418

Yazid narrated to us, Muhammad informed us, from Yahya ibn Abd al-Rahman ibn Hatib, who said: Abdullah ibn Umar said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The month is twenty-nine." And he clapped with his hands twice, then he clapped a third time, and he held back his thumb. Aisha said: May Allah forgive Abu Abd al-Rahman, for he has erred. The Messenger of Allah (peace and blessings of Allah be upon him) only stayed away from his wives for a month, then he came down after twenty-nine days. They said: O Messenger of Allah, you came down after twenty-nine days? He said: "The month can be twenty-nine."