Musnad Ahmad 27:1581

Hashim narrated to us, Ishaq bin Sa'id narrated to us, from his father, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "We are an unlettered nation; we do not calculate and we do not write. The month is like this, and like this, and like this." And he clenched his thumb on the third [time].