´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 ReferenceSunan an-Nasa'i / The Book of Fasting / Hadith: 2142
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:متفق عليه