Abu Dawud Book 8, Hadith Number 2752.
Chapter : Not known.
Narated By AbuRafi’ : The Quraysh sent me to the Apostle of Allah (PBUH), and when I saw the Apostle of Allah (PBUH), Islam was cast into my heart, so I said: Apostle of Allah, I swear by Allah, I shall never return to them. The Apostle of Allah (PBUH) replied: I do not break a covenant or imprison messengers, but return, and if you feel the same as you do just now, come back. So I went away, and then came to the Prophet (PBUH) and accepted Islam.