Bob is talking about HIS own source...NOT the first person to go around blabbing about Valerie, which was Joe Wilson for sure.
Bob had sources for his story. He has said there were two, and they were in the Administration but not in the White House. Many have speculated he could mean someone at the State Dept. and/or the CIA.
(He also checked with the CIA to see if they minded the info about Valerie appearing in his column and they did not mind. LOL, because later they asked for a Justice Dept. investigation into possible lawbreaking due to the supposed outing of a "covert" agent.)
Don't forget that Novak said this person was 'no partisan gunslinger'. This additional clue that the president knows who it was lends more creedance to it being Tenant. This would square with the President saying that anyone found to be involved in committing a crime would not be working in his administration. This assumes that the CIA was commiting a crime when it tried to influence a presidential election.