Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with him) said: Zinba' had a slave named Sandar, or Ibn Sandar. He found him kissing a slave girl of his, so he seized him, castrated him, and cut off his ears and nose. He came to the Prophet (peace and blessings be upon him), who sent for Zinba' and said: "Do not burden them with what they cannot bear; feed them from what you eat, and clothe them from what you wear. If you dislike a slave, sell him; and if you are pleased with him, keep him. And do not torment the creation of Allah." Then he (peace and blessings be upon him) said: "Whoever has been mutilated or burned with fire is free, and he is the freedman (mawla) of Allah and His Messenger." So the Prophet (peace and blessings be upon him) freed him. He said: "O Messenger of Allah, make a recommendation concerning me." He said: "I commend you to every Muslim."
Al-Muthanna bin al-Sabbah is da'if (weak) and is not used as proof. It has also been narrated from al-Hajjaj bin Artat from Amr in abridged form, and he is not used as proof; and from Sawwar Abu Hamzah from Amr, and he is not strong. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15950