A dog walker suffered a broken jaw and arm fractures in an attack in Doncaster.
The 53-year-old was walking along the old pit top, off Jenkinson Grove, Armthorpe, when he was targeted.
Three trail bikers - a man in his early 20s, a teenage boy and a teenage girl - were riding in the area just before the attack.
The man is reported to have assaulted the dog walker as he walked past him pushing his bike.
A South Yorkshire Police spokeswoman said: "During the incident, which happened at around 5.30pm on Sunday, July 23, a 53-year-old man was left with a broken jaw and two breaks to his arm.
"The man is said to have been walking his dog along the pit top, Jenkinson Grove, in Armthorpe, when he saw three people riding trail bikes in the area. Officers believe those riding the bikes were a man, a teenage boy and possibly a teenage girl.
"As the man was leaving the area, the man and the teenage boy walked past him pushing their bikes. It is then reported that the man, thought to be in his early 20s, assaulted the victim.
"Following enquiries in the area, officers have been working with the victim to produce an E-fit image of the alleged offender."
Anyone with information should call South Yorkshire Police on 101 or Crimestoppers, anonymously on 0800 555111.
Quote incident number 824 of July 23.