This was mentioned quite a lot in 2015, in my memory at least - especially in light of his business dealings in Russia but ultimately it's just conjecture. If it turned out to be true there's probably enough information in public to piece together a chronology but not without hindsight.
https://foreignpolicy.com/2018/12/21/how-russian-money-helpe...
To quote the man himself:
Russia has never tried to use leverage over me. I HAVE NOTHING TO DO WITH RUSSIA - NO DEALS, NO LOANS, NO NOTHING!