The man, believed to be in his 60s, was stabbed by a canal in Kings Norton, Birmingham, on Thursday.
Witnesses have reported that the man was stabbed twice by a thug who then robbed his bicycle. He remained in a serious condition in hospital on Thursday evening.
A West Midlands Police spokesman said two men were seen leaving the area in the direction of Brandwood Park.
Police and ambulance crews were sent to Chapel Walk, a cul-de-sac by the Worcester and Birmingham Canal, just after 1.50pm. Forensics teams remained at the scene on Thursday afternoon.
A West Midlands Police spokesman said: “We are appealing for information after a man was found with serious stab injuries in Kings Norton on Thursday afternoon. A man in his 60s was discovered in Chapel Walk at just after 1.50pm.
"It's believed he was also robbed of his bike after being attacked and he remains in a serious condition in hospital.
"Two men were seen leaving the area in the direction of Brandwood Park with the yellow bike, and we need your help to identify them.
"They are both white with one described as 6ft tall and the other understood to be around 5ft 6ins tall. We're keen to hear from anyone who has seen them with a bike of that colour."
A West Midlands Ambulance Service spokesman added: “We were called at 1.34pm to Chapel Walk in Kings Norton to reports of a stabbing.
“An ambulance, paramedic officer and the West Midlands CARE Team attended.
"One man was in a serious condition and received trauma care on scene before being taken on blue lights to hospital.”
Anyone with information is urged to call West Midlands Police on 101, quoting log number 2383 of October 15.