The Prophet ﷺ said, "Time has come full circle, returning to the form it had on the day Allah created the heavens and the earth. The year is twelve months, four of which are sacred. Three are consecutive, Dhul-Qadah, Dhul-Hijjah, and Muharram, and the fourth is Rajab of Mudar, which falls between Jumada and Shaban." The hadith continues.
عن النبي ﷺ قال: الزمان قد استدار كهيئة يوم خلق السموات والأرض السنة اثنا عشر شهرا، منها أربعة حرم، ثلاثة متواليات: ذو القعدة، وذو الحجة، والمحرم، ورجب مضر الذي بين جمادى وشعبان… الحديث.
Bukhari narrated it in the Book of Military Expeditions (4406), and Muslim in the Book of Qasama (1679:29), both through Abdul-Wahhab Thaqafi, who said, Ayub narrated to us, from Muhammad bin Sirin, from Ibn Abu Bakra, from Abu Bakra, and he related it.As for his saying, "Rajab of Mudar," there was a disagreement between Mudar and Rabia regarding the month of Rajab. Mudar considered Rajab to be this well known month that falls between Jumada and Shaban, while Rabia considered it to be Ramadan. This is why the Prophet ﷺ attributed it to Mudar. It is also said that this was because they honored it more than others did.