Volume 5 · Book 58 · Hadith 261
Narrated Al-Bara: The first people who came to us (in Medina) were Mus'ab bin 'Umar and Ibn Um Maktum. Then came to us 'Ammar bin Yasir and Bilal.
— Sahih Bukhari, Volume 5, Book 58: Merits of the Helpers in Madinah (Ansaar), Hadith #261