Musnad Ahmad 482:131

'Abd al-Samad ibn 'Abd al-Warith narrated to us, 'Abd al-'Aziz ibn Muslim narrated to us, Yazid ibn Abi Mansur narrated to us, from Dukhayn al-Hajari, from 'Uqbah ibn 'Amir al-Juhani, that a group of people approached the Messenger of Allah (may Allah bless him and grant him peace). He accepted the pledge of nine and withheld from one. They said: "O Messenger of Allah, you accepted the pledge of nine and left this one?" He said: "Indeed, he has an amulet upon him." So he inserted his hand and cut it off, then he accepted his pledge and said: "Whoever hangs an amulet has committed shirk (polytheism)."