Musnad Ahmad 27:688

'Abd al-Rahman narrated to us, from Sufyan, and Ishaq—meaning al-Azraq—who said: Sufyan narrated to us, from al-Aswad ibn Qays, from Sa'id ibn 'Amr, from Ibn 'Umar, from the Prophet, peace and blessings of Allah be upon him, who said: "We are an unlettered nation; we do not write, nor do we calculate. The month is thus and thus," until he mentioned twenty-nine. Ishaq said: And he clasped his hands three times, and held back his thumb on the third time.