Ubaydullah reported,

With this chain of narration, it states that the Messenger of Allah ﷺ mentioned Ramadan and said, "The month is twenty-nine days. The month is like this, and like this, and like this," and he said, "So calculate it," and he did not say "thirty."

Muslim 2501

Sahih

و حدثنا عبيد الله بن سعيد حدثنا يحيى بن سعيد

عن عبيد الله

بهذا الإسناد وقال ذكر رسول الله ﷺ رمضان فقال الشهر تسع وعشرون الشهر هكذا وهكذا وهكذا وقال فاقدروا له ولم يقل ثلاثين

مسلم 2501صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Muslim
Book
Fasting
Chapter
Obligation of fasting Ramadan upon sighting Hilal and breaking the fast upon sighting Hilal, and that if it is obscured at its beginning or end the month is completed as thirty days
Numbering
Darussalam 2501
Muslim 2501