Let φ: F1 --> F2 be an isomorphism field. Prove that φ(1) = 1. (That is, prove that φ must map the multiplicative identity of F1 to the multiplicative identity of F2.)

I know that there is a "1" element in F1 and a "1" element in F2, because they're fields. I also know that because φ is isomorphic, there exists a bijection from F1 to F2.

I need to prove that φ takes the "1" element in F1 to the "1" element in F2.

Can anyone help me from here? Thanks in advance.