Musnad Ahmad 27:367

Rawh narrated to us, Zakariya ibn Ishaq narrated to us, ‘Amr ibn Dinar narrated to us, that he heard ‘Abdullah ibn ‘Umar saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The month is like this, and like this, and like this." And he clenched his thumb on the third time.