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 (ﷺ) had said: Verily Allah deferred its sight, but if (the new moon) is hidden from you, then, complete its number (thirty)

English translation · Sahih Muslim