Sahih al-Bukhari 3934
Sahih al-Bukhari · Book 63: Merits of the Helpers in Madinah (Ansaar) · When did the Muslim calendar start?
Narrated Sahl bin Sa`d:
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.
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.