Musannaf Ibn Abi Shayba 26:539

Hatim ibn Isma'il narrated to us, from Ja'far, from his father, who said: Ali said: The foundling (al-manbudh) is free. If he wishes to form a client-relationship (wala') with the one who found him, he may form a client-relationship with him; and if he wishes to form a client-relationship with someone else, he may form a client-relationship with him.