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."
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 184
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 184