Authorities have arrested a man in connection with the face slashing of a tourist on an Upper West Side sidewalk a day ago, officials said Thursday. The 25-year-old was taken into custody on 85th Street. Police say the man has prior complaints against him. His mother went to the police precinct Wednesday night to identify him after seeing him on the news. The attack happened on West 86th Street between Columbus Ave. and Amsterdam Ave. just before 10:30 a.m., the FDNY said. Surveillance video showed a group of men walking and talking on the sidewalk when a man in a gray T-shirt and shorts runs from behind them, appearing to be holding a knife in his left hand. “It was completely random. The guy came up behind him and slashed him,” an unidentified neighbor said. “After he slashed this man, he almost cut off his ear completely.” The 55-year-old victim was recovering at the hospital after suffering being cut across his face, cheek and ear, a source told NBC New York. Some good Samaritans by the entrance to an apartment building stepped in to help the victim, as they sought to stop the bleeding. “He was sitting up in the doorway. One of his friends removed his shirt, holding against his head,” the neighbor said. “One of the eyewitnesses was pointing to a guy who just took off around the corner.” It’s unclear what exactly led to the stabbing. Just seconds after the man was seen on video running with a knife, the same man was seen running in the opposite direction while looking over his shoulder.