A.T. Robertson Commentary Mark 1:11

"And a voice came out of the heavens, Thou art my beloved Son, in thee I am well pleased." — Mark 1:11 (ASV)

Thou art (συ ε). So Lu 3:22. Matthew 3:17 has

this is (ουτος εστιν) which see. So both Mark and Luke have "in thee," while Matthew has "in whom."