Now that the new translation is about to be fully implemented, one hears the second form of the Penitential Act in a more faithful version than formerly – for the Latin is:
V/. Miserere nostri, Domine.
R/. Quia peccavimus tibi.
V/. Ostende nobis, Domine, misericordiam tuam.
R/. Et salutare tuum da nobis.
and the new English thereof is:
V/. Have mercy on us, O Lord.
R/. For we have sinned against you.
V/. Show us, O Lord, your mercy.
R/. And grant us your salvation.
(In both cases, I have added the versicle and response marks for clarity.)
But whence cometh this text?
The second versicle, used for perhaps a thousand years and more in the preparatory prayers at Mass, is of course from Psalm 84, but the first, while it sounds Scriptural, doesn't occur anywhere word-for-word in Holy Writ.
Initially, it does remind one of the words of the Lenten chant Attende, Domine, et miserere, quia peccavimus tibi (which, while not directly from Scripture, are hallowed by centuries of use, and have quite a nice tune to which they are sung – a pity they weren't simply inserted into the Mass if such an alternative to the Confiteor was actually necessary).
The nearest echoes I have found are as follows.
Baruch 3:2, especially in the Vulgate (still the official Latin version when the Novus Ordo Missæ was drawn up), is quite close:
Audi, Domine, et miserere, quia Deus es misericors: et miserere nostri, quia peccavimus ante te.
(In the Neo-Vulgate, as I call the Nova Vulgata, the allusion is obscured, as the phrase quia Deus es misericors: et miserere nostri is omitted, presumably because it is not in the Hebrew, and came into the Latin via the Septuagint through the process know as dittography, whereby the first phrase is redoubled. Also, the Neo-Vulgate changes the last two words of the verse into in conspectu tuo.)
Now as to finding exact verbal correspondences, the versicle is the same as those first few words of Psalm 122(123):3, Miserere nostri, Domine; whereas the reponse instead comes from the end of Jeremias 14:20, quia peccavimus tibi (in both cases, the words are the same in the Vulgate and New Vulgate).
Why was such a strange pastiche composed and inserted? Well, evidently the second versicle, Ostende nobis, Domine, was found worthy, and simply lifted from the versicles said by the priest at the conclusion of the prayers at the foot of the altar in the Extraordinary Form, but the versicle preceding it in that liturgy, Deus tu conversus, was evidently found "not quite suitable". Hence, the reformers made up a "better" one themselves. How curious.