Abu Hurairah reported,

The Messenger of Allah ﷺ said, "Fast when you see it, and break your fast when you see it. If the month is obscured from you, then count thirty days."

Azami 11955

Sahih

عن أبي هريرة قال

قال رسول الله ﷺ: صوموا لرؤيته، وأفطروا لرؤيته، فإن غمي عليكم الشهر فعدوا ثلاثين.

الأعظمي 11955صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Tafsir of the great Quran
Chapter
Virtues of Surah Baqarah
Section
His saying, "The month of Ramadan in which the Quran was revealed as guidance for the people and clear proofs of guidance and criterion. So whoever among you witnesses the month, let him fast it, and whoever is ill or on a journey, then a number of other days. Allah intends ease for you and does not intend hardship for you, and so that you may complete the number and glorify Allah for having guided you, and that you may be grateful."
Numbering
Jami al-Kamil 11955 (2nd edition)
Azami 11955