Who was the first of the Muhajirun to Medina?

The first to emigrate to Medina was the noble Companion Mus'ab ibn 'Umair. He had embraced Islam early on and kept his conversion secret. Then his family imprisoned him, but he escaped and emigrated to Abyssinia. After that, he returned to Mecca and emigrated to Medina. He participated in the battles of Badr and Uhud, where he was martyred.