Maliks Muwatta Book 29, Hadith Number 72.

Yahya related to me from Malik that he had heard that Ali ibn Abi Talib said about the two arbiters about whom Allah, the Exalted, said, “If you fear a breach between the two, appoint an arbiter from his people, and an arbiter from her people. If they desire to set things aright, Allah will make peace between them, surely Allah is Knowing, Aware,” (Sura 4 ayat 35), that the separation and the joining were overseen by the two of them.

Malik said, “That is the best of what I have heard from the people of knowledge. Whatever the two arbiters say concerning separation or joining is taken into consideration.”