I believe this is reported from the Prophet ﷺ, who said, "Two months do not decrease, the months of the two Eids, Ramadan and Dhul-Hijjah."
حدثنا إسماعيل عن خالد الحذاء
أحسبه عن النبي ﷺ قال شهران لا ينقصان شهرا عيد رمضان وذي الحجة
Its isnad is sahih, and it is found in Bukhari 4/124, no. 1912 (Fath), and Muslim 2/766, no. 1089, and Abu Dawud 2/307, no. 2323, all in the Fasting section, as well as Tirmidhi 3/66, no. 692, who graded it hasan, and Ibn Majah 1/531, no. 1659, and Bayhaqi 4/250, also in the Fasting section.