According to The New York Post:

The FBI has reviewed intercepted phone calls between national security adviser Michael Flynn and the Russian ambassador to the US and has found no evidence of wrongdoing, it was revealed Monday.

The calls were made in late December and picked up as part of routine electronic surveillance of Russian officials. They did not reveal any illicit ties between Flynn and Russia, according to the Washington Post.

The review of the calls was part of a wider probe into Russia's interference in the presidential election and hacking of the Democratic National Committee.