Musnad Ahmad 27:1669

Ubaydah ibn Humayd narrated to us, Al-Aswad ibn Qays narrated to me, from Sa’id ibn ‘Amr al-Qurashi, that ‘Abdullah ibn ‘Umar narrated to them, from the Prophet (may Allah bless him and grant him peace) that he said: "We are an unlettered nation; we do not write and we do not calculate. The month is thus, and thus, and thus." Then he withheld one [finger] in the third [instance].