Former Supreme Court Justice John Paul Stevens revealed Monday that he stepped down after experiencing a small stroke in 2010.

"Unbeknownst to me, I apparently had suffered a mini-stroke," Stevens, 98, writes in his forthcoming memoir, according to The New York Times.

He recalled how he stumbled over and mispronounced several words while delivering his dissent from the bench in the case of Citizens United v. FEC.

"That was it," he told the Times over the phone. “I made the decision that day. After I went to see the doctor, I sent a letter to the president right away.”

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In the landmark Citizens United decision, the Supreme Court in a 5-4 ruling found that the government was prohibited on free-speech grounds from restricting spending by corporations, nonprofits and unions on advertising for individual political candidates.

The decision unleashed new waves of political spending by outside groups in subsequent election campaigns.

Stevens authored a dissenting opinion joined by Justices Ruth Bader Ginsburg, Stephen Breyer and Sonia Sotomayor. He argued the ruling "threatens to undermine the integrity of elected institutions across the Nation."

Stevens's memoir, “The Making of a Justice: My First 94 Years,” is set for release next year.