source(RingMap) -- find the source ring for a map of rings

i1 : S = ZZ/10007[x, y, z];

i2 : R = ZZ/10007[t];

i3 : f = map(R,S,{t^3,t^4,t^5})

               3   4   5
o3 = map(R,S,{t , t , t })

o3 : RingMap R <--- S

i4 : source f

o4 = S

o4 : PolynomialRing