Maliks Muwatta Book 43, Hadith Number 3.

Section : The Blood-Money for Murder, When Accepted and the Criminal Act of the Insane.

Yahya related to me from Malik from Yahya ibn Said that Marwan ibn al-Hakam wrote to Muawiya ibn Abi Sufyan that a madman was brought to him who had killed a man. Muawiya wrote to him, “Tie him up and do not inflict any retaliation on him. There is no retaliation against a madman.”

Malik said about an adult and a child when they murder a man together, “The adult is killed and the child pays half the full blood-money.”

Malik said, “It is like that with a freeman and a slave when they murder a slave. The slave is killed and the freeman pays half of his value.”