Per Evan Drellich of Comcast SportsNet, the complaint came after Fister held the Yankees to four hits and one run over seven innings in Boston's 4-1 win on Sept. 1.
Drellich noted the YES Network broadcast showed Fister "walking around in the Sox dugout in the top of the eighth inning, once his outing was over, with the mouthpiece lodged around his ear."
This news comes one day after Michael S. Schmidt of the New York Times reported the Red Sox were using electronic devices "to illicitly steal hand signals from opponents' catchers in games against the second-place Yankees and other teams" after the Yankees filed an official complaint to MLB.
The report notes the Red Sox also filed a complaint to MLB alleging the Yankees use a camera from the YES Network broadcast of games to steal signs.
MLB commissioner Rob Manfred has not yet said how and if he will discipline the Red Sox for their use of electronic devices to steal signals.
The Red Sox enter play Wednesday leading the Yankees by 3.5 games atop the American League East.