Detectives are trying to trace a Good Samaritan who intervened in a robbery in Long Eaton this week.
A pair of muggers scarpered while they were robbing a man on the street, and heard calls from a passer by to leave him alone.
The incident took place sometime between 9.30pm and 10pm on Tuesday, October 20, when an 18-year-old man was walking along Wilsthorpe Road and realised two pedestrians and someone on a bicycle were behind him.
A spokesperson for Derbyshire police said: "The cyclist rode off, and as he approached the footpath which leads to Edale Close, one of the pedestrians grabbed him and he turned around. He was punched in the face and fell to the floor."
And as the robbers took his mobile and cash from his wallet, the victim heard someone telling the robbers to leave him alone, and the pair of attacked ran off.
Police are trying to trace the passer-by who intervened in case they have information which could help with the inquiry.
Officers said the man who punched him is described as black, in his 20s and about 6ft 2in tall with dreadlock hair. He was wearing grey jogging bottoms and a black fleece.
The victim is unsure as to the gender of the second robber but said they are white and wore black bottoms with a red stripe down the leg.
Anyone with information should call DC Kevin Beresford on 101, quoting crime reference number 45515/15, or Crimestoppers on 0800 555 111.