In Part One, I gave an overview of the penitential practices of bodily mortification (particularly, use of the hair shirt or biblical sackcloth). Here I provide the relevant Scriptures (RSV), so that Catholics may substantiate the biblical nature of their practices.

Use of Sackcloth as a Sign of Repentance or Penitence
Matthew 11:21 Woe to you, Chora’zin! woe to you, Beth-sa’ida! for if the mighty works done in you had been done in Tyre and Sidon, they would have repented long ago in sackcloth and ashes. (cf. Lk 10:13) …

http://www.ncregister.com/blog/darmstrong/more-biblical-support-for-bodily-mortification