Bukhari :: Book 7 :: Volume 63 :: Hadith 222
Narrated Ibn 'Umar:
The Prophet (holding out his ten fingers thrice), said, "The month is thus and thus and thus," namely thirty days. Then (holding out his ten fingers twice and then nine fingers), he said, "It may be thus and thus and thus," namely twenty nine days. He meant once thirty days and once twenty nine days.
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