An 18-year-old man needed hospital treatment after being attacked walking through Leicester.
He felt a sharp pain in his back as three men coming the opposite way walked past him on Great Central Way at about 1145 BST on Tuesday.
Shortly afterwards a passer-by told the victim his back was bleeding and he went to Leicester Royal Infirmary for treatment.
Police have appealed for witnesses to come forward.
One of the suspects is described as being about 5ft 9in tall, of medium build and wearing a black New York Yankees hooded top.