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."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحجر / Q11335