From the Libellus of Jordan of Saxony:
The Entrance of Brothers Jordan, Henry and Leo into the Order of Preachers
75. On Ash Wednesday, when the imposition of ashes reminds the faithful of their origin from and return to dust, we decided that a suitable way to begin the season of penance would be to fulfill the vow we had made to the Lord. Now none of our companions where we lived knew of our plan. So when Brother Henry left his lodgings and one of his companions asked him where he was going, he answered, "To Bethany." He did not understand what Henry meant, but later on he did, when he saw Henry enter Bethany, which means "the house of obedience." The three of us met at Saint-Jacques and, while the brethren were chanting "Immutemus habitu," we presented ourselves before them, much to their surprise, and, putting off the old man, we put on the new, thus suiting our actions to what they were singing.
Immutemur habitu, in cinere et cilicio: jejunemus et ploremus ante Dominum: quia multum misericors est dimittere peccata nostra Deus noster.
Let us change our garments for ashes and sackcloth: let us fast and lament before the Lord: for plenteous in mercy is our God to forgive our sins.