Ayesha reported,

When twenty-nine nights had passed which I had been counting, the Messenger of Allah ﷺ came in to me. She said, He began with me.I said, "Messenger of Allah, you swore that you would not come in to us for a month, yet you have come in after twenty-nine, which I have been counting."He replied, "The month is twenty-nine days."

Azami 4333

Sahih

عن عائشة، قالت

لما مضت تسع وعشرون ليلة أعدهن، دخل علي رسول الله ﷺ. قالت: بدأ بي. فقلت: يا رسول الله، إنك أقسمت أن لا تدخل علينا شهرا، وإنك دخلت من تسع وعشرين، أعدهن. فقال: إن الشهر تسع وعشرون.

الأعظمي 4333صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Fasting
Chapter
Obligation of fasting, its virtue, and its rulings
Section
Month can be twenty-nine days
Numbering
Jami al-Kamil 4333 (2nd edition)
Azami 4333