The return of Erling Haaland sparked Manchester City to dominant win that cut the gap at the top of the Premier League to five points.
After bottom the table Southampton, who knocked City out of the Carabao Cup in January, had started well, Haaland nodded home a Kevin De Bruyne cross just before half-time to open the scoring.
Jack Grealish fired in at the second attempt to double the lead in the second half, before Haaland acrobatically volleyed a 30th league goal of the season's from Grealish's cross.