Sahih Muslim Book 6, Hadith Number 2393.

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 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).