We do not seek the help of the polytheists against the polytheists

Khubayb bin Abd al-Rahman narrates from his father, and he from his grandfather (Sayyiduna Khubayb bin Yasaf (may Allah be pleased with him)), that the Messenger of Allah (peace and blessings be upon him) went out on a military expedition. Another man and I came to him (peace and blessings be upon him) before we had accepted Islam and said: "We feel ashamed that our people should take part in a battle while we stay behind." He (peace and blessings be upon him) said: "Accept Islam." We refused (at that time). So he (peace and blessings be upon him) said: "We do not seek the help of polytheists against polytheists." Then we accepted Islam and took part in the expedition with the Messenger of Allah (peace and blessings be upon him). There I killed a man, and he dealt me a wound. Later (as it happened) I married his daughter. She would often say: "May I never be without the man who adorned you with this sash (the wound, as a mark of honour)!" And I would say: "May you never be without the man who hastened your father to Hell!"
This hadith is sahih (authentic), but the two Shaykhs (Bukhari and Muslim) did not record it; and the grandfather of Khubayb bin Abd al-Rahman is a well-known Companion.

It is narrated from Sayyiduna Abu Humayd al-Sa'idi (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) went out (on a military expedition). When he (peace and blessings be upon him) had left Thaniyyat al-Wada' behind, a battalion appeared there. He (peace and blessings be upon him) asked: "Who are these people?" The Companions said: "These are Banu Qaynuqa' (Jews), who are of the clan of Abdullah bin Salam." He (peace and blessings be upon him) asked: "Have they accepted Islam?" They said: "No, rather they remain on their religion." He (peace and blessings be upon him) said: "Tell them to go back, for we do not seek the help of polytheists."