“When you see the new crescent then fast, and when you see it then stop fasting. If it is cloudy then fast thirty days.”
Sunan Ibn Majah Book 7, Hadith 18
← Previous Hadith ‘When you see the new crescent, fast, and when you see it, stop fasting. If it is cloudy then calculate it (as thirty days).” Ibn ‘Umar used to fast one day before the new crescent was seen.

Sunan Ibn Majah Book 7, Hadith 17 Next Hadith → ‘How much of the month has passed?” We said: “Twenty-two (days), and there are eight left.” The Messenger of Allah (ﷺ) said: “The month is like that, and the month is like that, (and the month is like that), three times, and he withheld one finger the last time.”

Sunan Ibn Majah Book 7, Hadith 19