3 “But Jesus, knowing in Himself that His disciples murmured at it,”— for they so said these things with themselves that they might not be heard by Him: but He who knew them in themselves, hearing within Himself—answered and said, “This offends you;” because I said, I give you my flesh to eat, and my blood to drink, this forsooth offends you. “Then what if you shall see the Son of man ascending where He was before?” What is this? Did He hereby solve the question that perplexed them? Did He hereby uncover the source of their offense? He did clearly, if only they understood. For they supposed that He was going to deal out His body to them; but He said that He was to ascend into heaven, of course, whole: “When you shall see the Son of man ascending where He was before;” certainly then, at least, you will see that not in the manner you suppose does He dispense His body; certainly then, at least, you will understand that His grace is not consumed by tooth-biting.
Source: Tractates on the Gospel of John (New Advent)