The NHL will fine the Dallas Stars for violating CBA rules by holding an optional practice during the league's holiday break, a league source told ESPN on Sunday.

The rules that the NHL and NHLPA agreed upon stipulate that all teams must be dark between Dec. 24 and Dec. 26. However, Stars coach Peter DeBoer revealed Friday, ahead of a game against the Minnesota Wild, that several Stars players were at the rink Thursday.

"I was at the rink yesterday," DeBoer told reporters Friday. "We had an optional, and of course, the young guys all show up for the optional like, you know, they're supposed to do."

The NHL launched an investigation after DeBoer's comments. According to sources, the Stars fully cooperated with the investigation.

In a memo Dec. 23, the NHL reminded teams of the rules, including that teams are not allowed to travel nor offer players ice time during the break.

There is some recent precedent. In 2022, the Toronto Maple Leafs were fined $100,000 for having players travel Dec. 26 for a game in St Louis on Dec. 27. The Philadelphia Flyers had a similar infraction in 2015, when they were also fined. The league sees this as a competitive disadvantage as teams that play Dec. 27 are expected to travel the morning of the game. The money for such a fine typically goes toward the NHL Foundation.

The NHL also institutes a trade freeze over the holidays, which was lifted at 12:01 a.m. on Saturday.

The NHL source said it was unclear whether the Stars had been notified Sunday afternoon. Dallas plays the Chicago Blackhawks on Sunday night.