Sayyiduna Khubayb bin Isaf (may Allah be pleased with him) said: I and a man from my people came to the Messenger of Allah (peace and blessings be upon him) before we had accepted Islam, and we said: "We feel ashamed that our people should take part in a battle in which we do not take part with them." The Messenger of Allah (peace and blessings be upon him) said to us: "Have you become Muslims?" We said: "No." He (peace and blessings be upon him) said: "I do not seek help from the polytheists against the polytheists." So we became Muslims and took part (in that battle) with the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4180