Sayyiduna Abu Mas'ud (may Allah be pleased with him) said that the first of the Muhajirun to come to Madinah was Sayyiduna Mus'ab bin Umayr (may Allah be pleased with him), and before the arrival of the Messenger of Allah (peace and blessings be upon him) he was the first to lead the Friday (Jumu'ah) prayer there.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14805