Sayyiduna Abu Rafi' (may Allah be pleased with him) says that a letter of Quraysh came to the Prophet (peace and blessings be upon him). The narrator says: When I saw the Prophet (peace and blessings be upon him), Islam became firm in my heart. I said: O Messenger of Allah (peace and blessings be upon him), I will not go back. The Prophet (peace and blessings be upon him) said: "I do not break a covenant, nor do I detain envoys. Go! If this same thing is in your heart, then come back." He says: I went back, then came (again) and became a Muslim. And Sayyiduna Abu Rafi' (may Allah be pleased with him) was a Copt.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18428