Hadith 2142
´It was narrated from Ibn 'Umar that the Prophet said:` "We are an unlettered Ummah, we do not use astronomical counting or computation. The month is like this, and this, and this," he did three times, showing it as twenty-nine.
Hadith 2143
´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.