It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that he said: "When the people established the reckoning of dates, they did not count from the commissioning of the Messenger of Allah (peace and blessings be upon him), nor from his death; rather, they counted only from his (peace and blessings be upon him) arrival in Madinah."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5889