“I want to thank my longtime friend Debbie Wasserman Schultz for her leadership of the Democratic National Committee over the past five years. I am grateful to Debbie for getting the Democratic Party to this year’s historic convention in Philadelphia, and I know that this week’s events will be a success thanks to her hard work and leadership. There’s simply no one better at taking the fight to the Republicans than Debbie–which is why I am glad that she has agreed to serve as honorary chair of my campaign’s 50-state program to gain ground and elect Democrats in every part of the country, and will continue to serve as a surrogate for my campaign nationally, in Florida, and in other key states. I look forward to campaigning with Debbie in Florida and helping her in her re-election bid–because as President, I will need fighters like Debbie in Congress who are ready on day one to get to work for the American people.”

That's not concrete evidence. It is well known that they are friends, but that does not necessarily mean there was any impropriety, especially of a magnitude large enough to tip the popular vote count or the pledged delegate count to Sanders.