Saku Koivu, who tied the game eight minutes into the third period, also had an assist, and Cristobal Huet stopped 29 shots, including 23 over the final 45 minutes in dousing the NHL's most potent offence. Huet's best save came with a minute left in overtime when he closed his pads in time to prevent Thomas Vanek's attempt to squeeze a shot through on a breakaway.
Jaroslav Spacek scored for the Sabres (18-3-2), who entered the game with a league-leading 101 goals, and were coming off two seven-goal games, including a 7-4 win over Toronto on Wednesday.
Souray's goal came on the transition and after Buffalo's Maxim Afinogenov had his shot blocked in the Montreal end. Alex Kovalev carried the puck up the right wing and fed a pass across to Souray, who beat Ryan Miller on the short side with a snap shot from the left circle.
Montreal won its third straight and improved to 5-1 in its past six while also snapping a four-game skid against Buffalo.
The Canadiens were particularly effective on the forecheck, preventing the high-flying Sabres from generating much momentum through the neutral zone. Montreal also kept most of Buffalo's shots from the outside.
Huet was sharp when necessary, winning his fifth consecutive start - a stretch in which he's allowed only eight goals.
The Sabres were fortunate to make it through regulation with the game tied after the Canadiens failed to score during a flurry of action in front with three minutes left.
Miller got a piece Craig Rivet's point shot, but the puck trickled behind him, settling dangerously in the crease. Sabres forward Daniel Paille dived into the crease to push the puck to safety and then appeared to deflect a shot from Sergei Samsonov from in close.
The Sabres opened the scoring 12 minutes into the second period when Brian Campbell hit Spacek open in the high slot.
Koivu, who turned 32 Thursday, then tied the game by capping a 3-on-1 rush. Driving up the right wing, he patiently avoided Miller's weak poke check and slipped the puck through the goaltender's legs.
Notes: Three of Koivu's five goals this season have come against Buffalo. ... The Sabres wrap up a stretch of 10 games in 17 days when they play at the New York Rangers on Sunday. Buffalo is 6-3 during that stretch. ... Canadiens D Janne Niinimaa was a healthy scratch for the fourth straight game. ... The Sabres were foiled in an attempt to score seven goals in three straight games for the first time since the final three games of the 1979-80 season.