Language:
كتاب مناقب الأنصار
63
Merits of the Helpers in Madinah (Ansaar)
(48)
Chapter: When did the Muslim calendar start?
(48)
باب التَّارِيخِ مِنْ أَيْنَ أَرَّخُوا التَّارِيخَ

Narrated Sahl bin Sa`d:

The Prophet's companions did not take as a starting date for the Muslim calendar, the day, the Prophet (ﷺ) had been sent as an Apostle or the day of his death, but the day of his arrival at Medina.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، عَنْ أَبِيهِ، عَنْ سَهْلِ بْنِ سَعْدٍ، قَالَ مَا عَدُّوا مِنْ مَبْعَثِ النَّبِيِّ صلى الله عليه وسلم وَلاَ مِنْ وَفَاتِهِ، مَا عَدُّوا إِلاَّ مِنْ مَقْدَمِهِ الْمَدِينَةَ‏.‏
Reference : Sahih al-Bukhari 3934
In-book reference : Book 63, Hadith 160
USC-MSA web (English) reference : Vol. 5, Book 58, Hadith 271
  (deprecated numbering scheme)