Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2393
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).