His campaign used stock footage supplied by their russian handlers just like kari lake and had to scramble to delete it after it came to light, most of his talking points mirror russian narratives, he constantly publicly defends russia, his position in us politics as spoiler on the left closely mirrors the strategy russia used with trump to dominate the republican party - only rfk is somehow less competent than trump.
It's just quite obvious from everything he does and says. I'm sure a little searching would turn up more examples.
I'm not responsible for providing basic, widely known, public information. It's like asking for proof that the us government supports israel. It's a matter of public record.