The Syrian Observatory for Human Rights said Mohammed Qatta, 15, had reportedly refused to give coffee to a customer, telling him, "Even if Muhammad comes back to life, I won't," ABC News reported Monday.
Extremist rebels driving past overheard the comment and took him away, the Aleppo Media Center said.
When they brought him back, the teen's head was covered with his shirt and his body bore marks of whipping. Speaking to a crowd that included the boy's parents, the rebels accused the teen of blasphemy and then killed him by shooting him in the mouth and neck.
The Islamists, and the group they belong to, had not been identified.