Musnad Ahmad 26:820

Yunus narrated to us, Mu'tamir narrated to us, from his father, from Sulayman al-A'mash, from Shaqiq ibn Salamah, from Abdullah ibn Mas'ud, who said: While we were walking with the Messenger of Allah (peace and blessings of Allah be upon him), he passed by some boys playing, among whom was Ibn Sayyad. The Messenger of Allah (peace and blessings of Allah be upon him) said: "May your hands be covered in dust; do you testify that I am the Messenger of Allah?" He replied: "Do you testify that I am the Messenger of Allah?" Umar said: "Let me strike his neck." The Messenger of Allah (peace and blessings of Allah be upon him) said: "If he is the one you fear, you will not be able to [kill] him."