Freeze a value in autofill

Does anyone know if there is a way to prevent one of the values from autofilling when using autofill? For example, if I have

=b2*a1 and use the plus to drag down to autofill, I get

=b3*a2 but what I want is 

=b3*a1 so when I drag down through hundreds of rows a1 stays static but b3 continues to autofill.