Sahih Muslim | Book 6 | Hadith Number 2366

Narrated by 'Ubaidullah
'Ubaidullah narrated on the authority of the same chain of transmitters that the Messenger of Allah (may peace be upon him) made a mention of Ramadan and said: The month may consist of twenty-nine days, and it may be thus, thus and thus, and (he further) said: Calculate it, but he did not say thirty.