Musnad Ahmad 27:1035

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Uqbah ibn Hurayth, I heard Ibn 'Umar saying: The Messenger of Allah (ﷺ) said: "The month is twenty-nine." And Shu'bah clapped his hands three times, and folded the thumb on the third time. 'Uqbah said: And I think he said: "And the month is thirty," and he clapped his palms three times.