Musnad Ahmad 1153:238

Yahya narrated to us, from Muhammad—meaning Ibn ‘Amr—who said: Yahya ibn ‘Abd al-Rahman narrated to me, from Ibn ‘Umar, from the Prophet, peace and blessings of Allah be upon him, who said: "The month is twenty-nine." They mentioned that to ‘Aishah, and she said: "May Allah have mercy on Abu ‘Abd al-Rahman; he only said: 'The month can be twenty-nine.'"