Musnad Ahmad 33:571

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, he said: I asked Jabir about a man who takes as a client the client of another man without his permission. He said: The Messenger of Allah (peace and blessings of Allah be upon him) wrote: "Upon every clan is their blood money (diya)." Then he wrote: "It is not permissible for a man to take as a client the client of a Muslim man without his permission."