Sahih al-Bukhari 4662
Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
The Statement of Allah: "Verily, the number of months with Allah is twelve months (in a year) so was it ordained by Allah on the Day when He created the heavens and the earth; of them four are sacred, (i.e., the 1st, the 7th, the 11th, and the 12th months of the Islamic calendar). That is the right religion; so wrong not yourself therein.. ." (V.9:36)
Narrated Abu Bakr: The Prophet (ﷺ) said, "Time has come back to its original state which it had when Allah created the Heavens and the Earth; the year is twelve months, four of which are sacred. Three of them are in succession; Dhul-Qa'da, Dhul-Hijja and Al-Muharram, and (the fourth being) Rajab Mudar (named after the tribe of Mudar as they used to respect this month) which stands between Jumad (ath-thani) and Sha'ban."