The Prophet ﷺ said, "Two months do not decrease, the two months of celebration, Ramadan and Dhul-Hijjah."
عن النبي ﷺ قال: شهران لا ينقصان، شهرا عيد: رمضان، وذو الحجة.
Bukhari narrated it in the Book of Fasting (1912), and Muslim narrated it in the Book of Fasting (1089), both of them via Mutamir bin Sulayman, from Ishaq bin Suwayd and Khalid Haddan, both of them from Abdul-Rahman bin Abu Bakra, from his father, and he related it. The wording given is Bukhari's.Regarding his saying "two months do not fall short," Ahmad said its meaning is, the two months of the two feasts do not fall short, meaning they do not both fall short together in a single year, Ramadan and Dhul-Hijjah, if one of them falls short the other is complete. Tirmidhi mentioned this following his citation of the hadith (692).And it has been said that its meaning is that they do not fall short in merit whether they are twenty nine or thirty days. These two views are well known from the early scholars, as Hafiz said in Fath al-Bari (4/125).