Sahih Muslim Book 6, Hadith Number 2392.

Chapter : It is immaterial whether the new moon is large (on account of appearing after thirty nights) or small (on account of appearing on the thirtieth night): and Allah defers it to make it suitable for sighting and if the sky is cloudy, then thirty (fasts) are to be completed.

Abu’l-Bakhtari reported: We went out to perform Umra and when we encamped in the valley of Nakhla, we tried to see the new moon. Some of the people said: It was three nights old, and others (said) that it was two nights old. We then met Ibn ‘Abbas and told him we had seen the new moon, but that some of the people said it was three nights old and others that it was two nights old. He asked on which night we had seen it; and when we told him we had seen it on such and such night, he said the Prophet of Allah (may peace be upon him) had said: Verily Allah deferred it till the time it is seen, so it is to be reckoned from the night you saw it.