President Trump on Friday renewed his attacks on football players who protest during the national anthem, after two members of the Miami Dolphins kneeled and one stood with a raised fist during the anthem Thursday ahead of the team’s first preseason game.

“The NFL players are at it again – taking a knee when they should be standing proudly for the National Anthem,” Trump wrote on Twitter. “A football game, that fans are paying soooo much money to watch and enjoy, is no place to protest.”

Trump also threatened that players would be suspended without pay if they continued to protest, though he has no authority over the National Football League’s disciplinary decisions. “Find another way to protest,” he wrote. “Stand proudly for your National Anthem or be Suspended Without Pay!”

Several football players, many of them African-American, have taken a knee or otherwise expressed solidarity during the national anthem since 2016, when then-San Francisco 49ers quarterback Colin Kaepernick began the protest movement to highlight police violence against people of color. Kaepernick, currently not signed by any NFL team, is pursuing a collusion case against the NFL, claiming he was blacklisted by team owners over his protests. The protests have been controversial, with some defending players’ right to free speech while others arguing the gridiron isn’t the place for political demonstrations.

Kaepernick on Friday tweeted about the Dolphins players — Kenny Stills, Albert Wilson and Robert Quinn — who engaged in the protests Thursday.

“My brother [Stills] continued his protest of systemic oppression tonight by taking a knee,” Kaepernick tweeted on Thursday. “Albert Wilson joined him in protest. Stay strong brothers!”

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Write to Billy Perrigo at billy.perrigo@time.com.