Sahih Muslim | Book 6 | Chapter 6

Narrated by Abu'l-Bakhtari
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.

Narrated by Abu'l-Bakhtari
Abu'l-Bakhtari reported: We saw the new moon of Ramadan as we were at Dhit-i-'Irq. We sent a man to Ibn Abbas (Allah be pleased with both of them) to ask him (whether the sighting of a small moon had something of the nature of defect in it). Upon this Ibn 'Abbas (Allah be pleased with both of them) said that the Messenger of Allah (may peace be upon him) had said: Verily Allah deferred its sight, but if (the new moon) is hidden from you, then complete its number (thirty).