Al-Waqidi, mentioning the martyrs of Uhud, says that Mujadhdhar bin Ziyad (may Allah be pleased with him) was killed treacherously by al-Harith bin Suwayd. His story is as follows: Mujadhdhar (may Allah be pleased with him) had killed his (al-Harith's) father, Suwayd bin al-Samit, in the Jahiliyyah. When the Prophet (peace and blessings be upon him) came to Madinah, Mujadhdhar and al-Harith bin Suwayd (may Allah be pleased with them both) became Muslim and took part in Badr, and al-Harith tried to kill Mujadhdhar (may Allah be pleased with him) that day too but could not. When the day of Uhud came and the Muslims began to fall back, al-Harith came from behind and killed Mujadhdhar (may Allah be pleased with him). The Prophet (peace and blessings be upon him) came to Madinah and then set out for Hamra al-Asad. When he returned, Jibril (peace be upon him) came and told him (peace and blessings be upon him) that Mujadhdhar (may Allah be pleased with him) had been killed by al-Harith, and treacherously at that, so he (peace and blessings be upon him) ordered his execution. When he (peace and blessings be upon him) came to Quba and saw al-Harith there, he (peace and blessings be upon him) called Uwaym bin Sa'idah (may Allah be pleased with him) and ordered: "Al-Harith is coming toward the door of the mosque; kill him in retaliation for Mujadhdhar bin Ziyad (may Allah be pleased with him), whom he killed treacherously on the day of Uhud." Uwaym (may Allah be pleased with him) seized him, and he said: "Just let me speak to the Messenger of Allah (peace and blessings be upon him) once." Uwaym (may Allah be pleased with him) refused. As he (peace and blessings be upon him) moved to mount, he began saying: "By Allah, I did kill him, O Messenger of Allah! But I did not kill him because I had turned away from Islam or had doubts about it; rather I killed him out of devilish zeal. Now I repent to Allah; I will pay his blood money, free a slave, fast for two months, and feed sixty poor people, and I repent to Allah." And he took hold of the reins of his (peace and blessings be upon him) mount. The Banu Mujadhdhar (may Allah be pleased with them) were also present, but they were silent; the Messenger of Allah (peace and blessings be upon him) had said nothing to them. When his talk grew long, he (peace and blessings be upon him) ordered Uwaym (may Allah be pleased with him): "Uwaym! Take him and kill him." So Uwaym (may Allah be pleased with him) struck his neck.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16061