Islamist rebel militiamen in Syria publicly executed a 14-year-old boy for mentioning the name of Islam’s Prophet Muhammad in a way they deemed blasphemous, according to the activist Syrian Observatory for Human Rights.

The Observatory first reported Muhammad Qatta’s death in the Shaar neighborhood of Aleppo yesterday, and said it released two videos showing his parents today to authenticate it. In one video, a woman identified as the boy’s mother, said she witnessed his death. “His blood flowed in front of me,” she said, her head covered with a large flowing scarf.

The Syrian National Coalition, the main political opposition, said in an e-mailed statement that if the report was true, the killing would be considered “a crime against humanity and a blatant violation of human rights.”

The woman in the video said Qatta was 14 and sold coffee to help support the family. A man identified as his father said in another video that his son was telling someone who wanted to pay for coffee later that he cannot lend anyone anything, even if “Muhammad comes back to earth.”

The man in the video said three gunmen overheard Qatta and asked him why he was saying something blasphemous. He said they took his son away for 30 minutes and then came back with him. He was at home praying when he heard the three shots that killed Qatta, he said. The family has filed a complaint with a sharia committee, the man said....