Yes, there's tons of evidence regarding the fact that the Soviet Union got lots of espionage information about nuclear weapons during World War II and used it towards their nuclear program. The evidence includes:

• confessed spies (Klaus Fuchs and David Greenglass being the major ones, who confessed in the 1950s; Ted Hall somewhat confessed later in life)

• decrypted communications (the Venona project, released in the 1990s), which indicated many unconfessed spies as well (including — famously — Julius Rosenberg, who along with his wife, Ethel, was sent to the electric chair for this in 1953)

• actual files from the Soviet archives (released in the 1990s-2000s) that make it quite clear how they did (and didn't) use the espionage information (an example of the kind of stuff that has been released)

Anyway this is a common-enough claim about the Soviet nuclear program that I am surprised you have not encountered it before. Much of the time when scholars talk about this today, it is to emphasize that just having a bunch of spy data does not necessarily mean that it accelerates your bomb program (aside from the fact that spy data is not sufficient, by itself, to make a bomb, it also depends on how one uses the data, and the Soviets did not trust the spy data enough to just rely on it alone).

A very nice volume that discusses the usage of espionage in the Soviet program using recently-released sources from the Soviet side of things is Michael Gordin's Red Cloud at Dawn (Farrar, Straus & Giroux, 2009).