When you see the new moon, observe fast, and when you see it (again) then break it, and if the sky is cloudy for you, then calculate it.
Sahih Muslim Book 13, Hadith 10
← Previous Hadith The month (of Ramadan) may consist of twenty nine days; so when you see the new moon observe fast and when you see (the new moon again at the commencement of the month of Shawwal) then break It, and if the sky is cloudy for you, then calculate it (and complete thirty days).

Sahih Muslim Book 13, Hadith 9 Next Hadith → The month may consist of twenty-nine nights. So do not fast till you have sighted it (the new moon) and do not break it till you have sighted it, except when the sky is cloudy for you, and if it is so, then calculate it.

Sahih Muslim Book 13, Hadith 11