The incident happened on the M train at the 42 St-Bryant Park station just before 9:20 a.m.
Surveillance footage shows the suspect approaching a rider sitting on the train and attempting to take his cell phone.
The victim resisted and the robber punched him.
The subway rider then pulled a knife and slashed the man in the arm.
The slashed man was taken to Bellevue Hospital in stable condition. Investigators are interviewing the slashed suspect.
Police say he had an accomplice who fled the scene.
The rider retained possession of his cell phone the entire time and continued on his commute.
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