Musnad Ahmad 27:568

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from al-Aswad ibn Qays, I heard Sa'id ibn 'Amr ibn Sa'id narrating that he heard Ibn 'Umar narrating 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." And he tied his thumb in the third [instance]. "And the month is thus, and thus, and thus," meaning the completion of thirty.