Musnad Ahmad 27:1863

Abu Kamil narrated to us, Ibrahim narrated to us, Ibn Shihab narrated to us, from Salim, from Abdullah ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you see the crescent, then fast, and when you see it, then break your fast. But if it is obscured from you, then estimate it."