ENOfficial
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.
Source: sunnah.com
Other translations
Arabic
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.
sunnah.com