From Sahih Bukhari (Hadith 3197):
Narrated Abu Bakra:The Prophet (ﷺ) said. "(The division of time has turned to its original form which was current when Allah created the Heavens and the Earths. The year is of twelve months, out of which four months are sacred: Three are in succession Dhul-Qa' da, Dhul-Hijja and Muharram, and (the fourth is) Rajab of (the tribe of) Mudar which comes between Jumada-ath-Thaniyah and Sha`ban
Reference: Bukhari 3197