Musnad Ahmad 27:590

Muhammad ibn Ja'far and Bahz narrated to us, they said: Shu'bah narrated to us, from Jabalah ibn Suhaym, he said: Bahz informed me, he said: 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. Muhammad ibn Ja'far said in his narration: Meaning his saying: "Twenty-nine."