Praise be to Allah,
Wiping over Khuffs is an established sunnah. This can be proven from the following hadith.
‘Mughira b. Shu’ba reported: The Messenger of Allah (ﷺ) went out for relieving himself. When he came back, I brought him a jar (of water) and poured water upon his hands and He washed his face. He tried to wash his forearms, but the (sleeves of the) gown were tight. He, therefore, brought them out from under the gown. He then washed them, wiped his head, and wiped his socks, and then prayed’.[i]
‘Urwa b. Mughira reported his father having said: I was one night with the Messenger of Allah (ﷺ) on a journey. He said to me: Have you any water with you? I said: Yes. He (the Holy Prophet) came down from his ride and went on till he disappeared in the darkness of night. He then came back and I poured water for him from the jar. He washed his face, He had a woolen gown on him and he could not bring out his forearms from it (i. e. from its sleeves) and consequently he brought them out from under his gown. He washed his forearms, wiped over his head. I then bent down to take off his socks. But he said: Leave them, for my feet were clean when I put them in, and he only wiped over them.’[ii]
Allah Knows best.
[i] Sahih Muslim 274e
[ii] Sahih Muslim 274f