Allah Almighty says: "Then if the one who owes the debt is of deficient understanding, or weak, or unable to dictate himself, then let his guardian dictate with justice." [Surah al-Baqarah: 282]
Imam al-Shafi'i (may Allah have mercy on him) said: "So Allah Almighty established guardianship over the one of deficient understanding, the weak, and the one unable to dictate, and commanded his guardian to dictate on his behalf."
Urwah bin al-Zubayr (may Allah have mercy on him) said: Abdullah bin Ja'far (may Allah be pleased with them both) bought a piece of land for six hundred thousand dirhams, and Sayyiduna Ali and Sayyiduna Uthman (may Allah be pleased with them both) intended to place a restriction on his financial dealings. He said: I met Sayyiduna al-Zubayr (may Allah be pleased with him), and he said: No one has made a cheaper purchase than the one you have made. Then Abdullah (may Allah be pleased with him) mentioned the restriction on him, and he said: If I had the money, I would be your partner. He said: I will lend you half the money. He said: Then I am your partner. He said: Then Sayyiduna Ali and Sayyiduna Uthman (may Allah be pleased with them both) came to the two of them while they were discussing. Sayyiduna al-Zubayr (may Allah be pleased with him) asked: What are you discussing? They mentioned the restriction on Abdullah (may Allah be pleased with him). Sayyiduna al-Zubayr (may Allah be pleased with him) said: Will you place a restriction on a man whose partner I am? They said: No, by my life! He said: I am his partner. Then the restriction was dropped.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / 11335