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." (DE)
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 184 (DE)
Arabic reference (DE): Book 65, Hadith 4662 (DE)