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.
Reference: Sunan an-Nasa'i 2140 In-book : Book 22, Hadith 51 English translation : Vol. 3, Book 22, Hadith 2142
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