I've never been completely sure which things Dr. McGrath is almost certain Jesus said, but he provided one example in a recent comment. "And given the evidence, I certainly do think that there are a small number of things which it is very probable that Jesus said - the use of abba in reference to God being a good example." "Abba" is an Aramaic word meaning "father" that appears three times in the New Testament at Mark 14:36, Romans 8:15, and Galatians 4:6. It is only the first passage that attributes use of the word to Jesus while he is praying in the Garden of Gethsemane.
I found the explanation for McGrath's confidence in a 2008 post titled What Jesus Said and Did: 1) Prayer in Gethsemane
Paul already seems to have associated the address of God as Father in Aramaic with Jesus’ relationship to God as son. The only appearances of this Aramaic word in the New Testament are in the earliest Gospel and in Paul’s letters. And there is no reason Paul would have used the term in writing to non-Aramaic-speaking Christians other than that the term already had some significance for them, presumably in connection with Jesus, the object of their faith and devotion.That's a pretty big leap there. It is perfectly reasonable to suppose that Christian use of the word "Abba" predates Paul's letters, but that doesn't give us any reason to suppose that it was Jesus who first used it. It could have been Peter or James or any other Aramaic speaker who introduced the word into the Church's vocabulary. It might even have been Paul himself at some time prior to writing the epistles. There is nothing in Paul's letters that gives us a clue as to how the word "Abba" came to have significance for Greek speaking Christians.
The only thing pointing to Jesus as the responsible party is Mark 16:36, where Jesus prays “Abba! Father! All things are possible for You; remove this cup from Me; yet not what I will, but what You will.” However, this could just be Mark attributing to Jesus a usage that was already common among Christians. I find it interesting that Jesus' only use of the word comes in the Garden of Gethsemane when there would have been no witnesses as he was praying alone while his disciples slept. Had tradition preserved some genuine memory of Jesus using the word, I might expect Mark to put the word on his lips in some more public setting.
Is it possible that Jesus actually used the word "Abba" to describe his relationship with God? Sure. Is it "very probable" based on the available evidence? Not a chance.