The Father has committed Himself fully in the Son. Whatever the Son speaks is only what the Father has told Him to say (3:35, 4:34). This was prophesied in (Deut 18:18-19).

John 3:35  The Father loves the Son and has placed everything in his hands.

John 4:34  “My food,” said Jesus, “is to do the will of him who sent me and to finish his work.

Deuteronomy 18:18-19  I will raise up for them a prophet like you from among their brothers; I will put my words in his mouth, and he will tell them everything I command him.  If anyone does not listen to my words that the prophet speaks in my name, I myself will call him to account.

Some of the things that the Father has placed into the hands of His Son are:

• the Spirit (3:34),
• the work He accomplished (17:4),
• the works He did (5:36, 14:31),
• His message (12:49, 17:8),
• His authority (17:2),
• His name (17:11),
• His glory (17:22-24),
• His disciples (6:37-39, 10:29, 17:6, 12, 24, 18:9),
• the cup He must drink (18:11),
• all judgment (5:22),
• life in himself (5:26),
• and power over all flesh (17:2).

“say” and “speak” (KJV) are translated as “what to say and how to say it” in the NIV. The words refer to content and manner of delivery. Note that the command of the Father to Jesus was not only “what” to say, but “how” to say it. What we have to say to people is important, because we have the good news of salvation, but it is equally important that we say it the right way. Many a Christian has turned unbelievers away from the possibility of believing the truth because they attacked them with the message instead of loving them with it. How do we know exactly how to witness to others? The same way Jesus did — by asking the Father and then doing what He says.