Challenging someone's credibility when that person's credibility is relevant to whether or not one should believe him or her does not constitute an ad hominem attack.  Someone ought to tell John Bolton, whose credibility in matters of foreign affairs ought to have suffered for the Iraq debacle.  Well, someone did.  An interviewer asked whether Americans would be rightly skeptical of the administration and its minions'  claims about Iran's military intentions.  And here is how Bolton responded:

Absolutely not! And by the way, the credibility point is an ad hominem reference. And certainly ad hominem attacks in American politics are nothing new.  But to address the merits of the argument requires a response on the merits, not an ad hominem attack.

No it's not an ad hominem attack.  An ad hominem argument uses irrelevant information about the person making an argument to discredit them–whether one should believe Bolton's doomsday rhetoric about Iran is certainly relevant.  

You can hear the audio here.