Sunan an-Nasa'i 2141

It was narrated from Saeed bin 'Amr bin Saeed bin Abi Al-As, that:he heard Ibn 'Umar narrate that the Prophet said; "We are an unlettered Ummah; we do not use computation. The month is like this, and this, and this," and he held down his thumb the last time. "And the month is like this, and this, and this," completing thirty