And the Father who sent me has himself testified concerning me. You have never heard his voice nor seen his form, John 5:37 NIV
The Father spoke of His Son when Jesus came out of the water in baptism, which the Jewish leaders chose not to recognize as such. The Father also spoke of His Son during His transfiguration, which was only witnessed by the disciples Peter, James and John
Christ here quite possibly speaks of hearing in the sense of obedience. The Jewish leaders certainly did not obey that which came from the Father as they would not accept Christ for who He was.
No one has seen the Father except for the Son who came to us from the Father.
Let us be obedient to our Lord. We must hear Him just as He had heard the Father.