Mukhtasar Muslim (Abridged) 574 — Fasting is based on sighting

Narrated Abu Huraira (may Allah be pleased with him): The Messenger of Allah (peace and blessings be upon him) mentioned the moon and said: When you see the moon, then fast. And when you see it (again), then break your fast. But if clouds come, then complete thirty days of fasting (and then celebrate Eid).