Narrated AbuBakrah: The Prophet (ﷺ) gave a sermon during his hajj and said: Time has completed a cycle and assumed the form of the day when Allah created the heavens and the earth. The year contains twelve months of which four are sacred, three of them consecutive, viz. Dhul-Qa'dah, Dhul-Hijjah and Muharram and also Rajab of Mudar which comes between Jumadah and Sha'ban.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا إِسْمَاعِيلُ، حَدَّثَنَا أَيُّوبُ، عَنْ مُحَمَّدٍ، عَنِ ابْنِ أَبِي بَكْرَةَ، عَنْ أَبِي بَكْرَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم خَطَبَ فِي حَجَّتِهِ فَقَالَ " إِنَّ الزَّمَانَ قَدِ اسْتَدَارَ كَهَيْئَتِهِ يَوْمَ خَلَقَ اللَّهُ السَّمَوَاتِ وَالأَرْضَ السَّنَةُ اثْنَا عَشَرَ شَهْرًا مِنْهَا أَرْبَعَةٌ حُرُمٌ ثَلاَثٌ مُتَوَالِيَاتٌ ذُو الْقَعْدَةِ وَ ذُو الْحِجَّةِ وَالْمُحَرَّمُ وَرَجَبُ مُضَرَ الَّذِي بَيْنَ جُمَادَى وَشَعْبَانَ " .
sunnah.com