Musnad Ahmad 27:1087

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Jabalah, I heard Ibn 'Umar saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The month is like this," and he clasped his fingers twice, and in the third [time] he folded the thumb, meaning his saying: twenty-nine.