Mukhtasar Bukhari (Abridged) 924 — Saying "Ramadan" or "month

Narrated Ibn Umar (may Allah be pleased with them) that: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "When you see the (crescent of) Ramadan, start fasting, and when you see the (crescent of) Eid al-Fitr, stop fasting. But if the sky is cloudy (i.e., overcast), then estimate for it (i.e., complete thirty days)."