Merits of the Helpers in Madinah (Ansaar)

Bukhari :: Book 5 :: Volume 58 :: Hadith 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.

Source materials are from the University of Southern California MSA site
Hadith eBooks converted from Imaan Star