It is narrated from Sayyiduna Abu Rafi' (may Allah be pleased with him) that he came to the Messenger of Allah (peace and blessings be upon him) carrying a letter from Quraysh. He said: When I saw the Prophet (peace and blessings be upon him), Islam was cast into my heart, and I said: O Messenger of Allah! By Allah, I will never go back to them. The Messenger of Allah (peace and blessings be upon him) said: "I do not break a covenant, nor do I detain envoys. But go back, and then if what is in your heart now is still in your heart, come back." So I went back to them, then I came to the Messenger of Allah (peace and blessings be upon him) and accepted Islam.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 959