Sahih Muslim | Book 6 | Hadith Number 2376

Narrated by Aswad b. Qais
Ibn 'Umar(may Allah be pleased with both of them)reported Allah's Apostle (may peace be upon him) as saying: We are an unlettered people who can neither write nor count. The month is thus, and thus, folding his thumb when he said it the third time. This hadith has been narrated on the authority of Aswad b. Qais with the same chain of transmitters, but herein no mention has been made of the other month (consisting of) thirty days.