Scoth gambit refutation ?

I had a strange feeling on an online fast game the other day when my opponent played 5.Qf6 on the scotch gambit (1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.Nxd4 Nxd4 5.Qxd4 Qf6). This doesn't seem to be a book move to my mind. I am not sure however of the ebst response. Any suggestion is welcome