The Wikipedia link I posted earlier, which Bob said he had already read, is not the Watergate page but the Nixon Tapes page. It includes an audio file of the smoking gun tape.
Link
Here's a bit of the smoking gun tape, edited down from a
transcript on the Watergate.com website:
In other words, contrary to Nixon's testimony in Congress, he was very aware of the Watergate break-in and was actively involved in covering it up. That he was trying to get the CIA to obstruct the FBI despite the fact there was no national security issue involved. If they had tried to place bugs in the DNC Watergate offices because they had information of criminal activity there, or that they had information of something that threatened national security, that would have been different. But this was all politics. Basically Republican politics.
That was clearly understood at the time. Essentially, everything this tape revealed about what Nixon knew -- and when he knew it -- totally contradicted his previous statements. This is from the Wiki Nixon Tapes: