I have never known a self-proclaimed atheist to actually say as much. This is purely a line toted about by those who wish to paint "atheists" in a certain light ("they're just as irrational as we!").
It's rather uncommon for any follower of faith to preface their beliefs with "I believe". If they preface them at all, they're much more likely to use "I know", or "$authority says".
All of them that recite the Nicene Creed or some derivative...
When given the choice between
"I believe that God doesn't exist"
and
"I don't believe that God exists"
>99% of atheists would choose the latter as better representing their personal stance on the matter. Further, almost all would additionally agree that if there were evidence for God's existence, they would believe.
The concept of "faith" is not one that resonates with atheists. Christians however, love to pretend that disbelieving requires just as much magical thinking as believing, and this is a classic example.