Musnad Ahmad 27:733

Yahya ibn Sa'id narrated to us, from Muhammad ibn 'Amr, Yahya ibn 'Abd al-Rahman informed me, from Ibn 'Umar, from the Prophet, may God bless him and grant him peace, who said: "The month is twenty-nine." They mentioned that to 'A'ishah, and she said: "May God have mercy on Abu 'Abd al-Rahman. Did the Messenger of God, may God bless him and grant him peace, abandon his wives for a month, so it descended as twenty-nine?" It was said to him, so he said: "Indeed, the month may be twenty-nine."