Sahih Muslim Book 19, Hadith Number 4472.

Chapter : Disapproval of seeking help from a disbeliever on a military campaign.

It has been narrated on the authority of ‘Aisha, wife of the Holy Prophet (may peace be upon him), who said: The Messenger of Allah (may peace be upon him) set out for Badr. When he reached Harrat-ul-Wabara (a place four miles from Medina) a man met him who was known for his valour and courage. The Companions of the Messenger of Allah (may peace be upon him) were pleased to see him. He said: I have come so that I may follow you and get a share from the booty. The Messenger of Allah (may peace be upon him) said to him: Do you believe in Allah and His Apostle? He said: No. The Messenger of Allah (may peace be upon him) said: Go back, I will not seek help from a Mushrik (polytheist). He went on until we reached Shajara, where the man met him again. He asked him the same question again and the man gave him the same answer. He said: Go back. I will not seek help from a Mushrik. The man returned and overtook him at Baida’? He asked him as he had asked previously: Do you believe in Allah and His Apostle? The man said: Yes. The Messenger of Allah (may peace be upon him) said to him: Then come along with us.