Musannaf Ibn Abi Shayba 4:742

Yazid ibn Harun narrated to us, from Muhammad ibn ‘Amr, from Yahya ibn ‘Abd al-Rahman ibn Hatib, from Ibn ‘Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The month is twenty-nine." Then he clapped his hands together twice, and the third time he clapped, he held back his thumb.