To be practical ,how one could give a formal proof of the following theorem?

if a>=0 and b>=0 then sqrt(ab)= sqrt(a)\*sqrt(b).

i.e if a,b are greater or equal to zero then the sqrt of ab is equal to the product of sqrt of a multiply by the sqrt of b.

I have an idea how to do that proof formally,but i would like to see if anyone else could do that