Sahih Bukhari
Merits of the Helpers in Madinah (Ansaar)
Volume: 005 / Book Number: 058 / Hadith Number: 271
Narrated Sahl bin Sad:
The Prophet's companions did not take as a starting date for the Muslim calendar, the day, the Prophet had been sent as an Apostle or the day of his death, but the day of his arrival at Medina.