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Kevin Federline claims Britney Spears once slapped him following an argument over weed

Spears has slammed Federline's forthcoming memoir, criticizing him for trying to "profit off my pain"

His book, You Thought You Knew , is out Oct. 21

Kevin Federline is reflecting on a pivotal moment in his marriage to Britney Spears that rocked their foundation for good.

Federline, 47, wrote in his memoir You Thought You Knew about a day in which he and Spears, 43, got into an argument over his smoking weed, and she allegedly slapped him while holding their young son.

The former dancer — who was married to Spears from 2004 to 2007 — wrote that he was in the studio working on his music when a friend showed up to drop off an ounce of weed, something he says was relatively commo

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