Lyrics in parentheses are background vocals sung by Chaka Khan

Clay Aiken: What is a summer really
But a tepid month or three?
Ferb: Clay Aiken?
Phineas: Yeah. I hired a stunt singer, what do you think?
Clay Aiken: If you agree with Buford
That's all it will ever be

And we don't need to break the laws of physics
To make a day that's longer than a day
We can follow that old sun 'round the circumference of the globe
And stop all the naysayers from naysay... ing

'Cause I believe we can
And that's the measure, the measure of a man
It may sound far-fetched, this thing we've got planned
But I believe we can

I believe we can (Ooh, I believe, I believe we can)
And that's the measure, the measure of a man
(Oh, that's the measure of a man)
Phineas: (to Ferb) Chaka Khan. Nice.
We'll make it back here (We'll make it back here)
to where we began...(Oh...)
Phineas: Or at least that's the plan!
Chaka Khan: Of course, first you're gonna have to fix this wing, yeah!
Hey where's Perry?

http://phineasandferb.wikia.com/wiki/I_Believe_We_Can