Abdullah bin Umar reported,

The Messenger of Allah ﷺ mentioned Ramadan and said, "Do not begin fasting until you see the crescent moon, and do not break the fast until you see it. If it is hidden from you by clouds, then estimate its appearance."

Azami 11954

Sahih

عن عبد الله بن عمر

أن رسول الله ﷺ ذكر رمضان، فقال: لا تصوموا حتى تروا الهلال، ولا تفطروا حتى تروه، فإن غم عليكم فاقدروا له.

الأعظمي 11954صحيح
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 11954 (2nd edition)
Azami 11954