Musnad Ahmad 33:643

Musa narrated to us, Ibn Lahi'ah narrated to us, from Abu al-Zubayr, who said: I asked Jabir about a man who forms a client-relationship (wala') with the clients (mawali) of another man without his permission. He said: The Prophet, peace and blessings of Allah be upon him, wrote: "Upon every clan is their blood money (diya/aqil)." Then he wrote: "It is not permissible for a man to form a client-relationship with the clients of another man without his permission."