The nearest I can find is "And when the sacred months have passed, then kill the polytheists wherever you find them and capture them and besiege them and sit in wait for them at every place of ambush. But if they should repent, establish prayer, and give zakah, let them [go] on their way. Indeed, Allah is Forgiving and Merciful."

This is really fairly unlike Bobad's alleged quote of "The ambassador answered us that [their right] was founded on the Laws of the Prophet, that it was written in their Koran, that all nations who should not have answered their authority were sinners, that it was their right and duty to make war upon them wherever they could be found, and to make slaves of all they could take as prisoners, and that every Mussulman who should be slain in battle was sure to go to Paradise."

Here is an explanation of "zakah" - http://www.uwt.org/site/article.asp?id=172