Augustine of Hippo — as excerpted in the Catena Aurea on Mark 9:43-50

“(de Con. 4. 6) Mark relates that the Lord said these things consecutively, and has put down some things omitted by every other Evangelist, some which Matthew has also related, others which both Matthew and Luke relate, but on other occasions, and in a different series of events. Wherefore it seems to me that our Lord repeated in this place discourses which He had used in other places, because they were pertinent enough to this saying of His, by which He prevented their forbidding miracles to be wrought in His name, even by him who followed Him not together with His disciples.”