Latin is not among the 115 languages into which the Book of Mormon has been translated, so I had to do this myself. The original prayer of Alma the Elder at the Waters of Mormon, just prior to baptizing Helam, is:
O Lord, pour out thy Spirit upon thy servant, that he may do this work with holiness of heart (Mosiah 18:12).
Here is my Latin rendition:
Effunde Spiritum tuum, Domine, super servum tuum, ut opus hoc faciat in sanctitate cordis. Amen.
I spent quite a lot of time fiddling with different word orders, and I'm fairly confident that this one flows the best. My only real liberty with the text was to translate with holiness of heart as in sanctitate cordis (rather than cum sanctitate cordis). I have no real explanation for this, other than that my "ear for Latin" (such as it is, trained only on the Rosary and the Vulgate Psalms) demands it. A Google search shows that cum sanctitate cordis is an attested Latin expression but that in sanctitate cordis is about 100 times more common, so I suppose I'll take that as confirmation of my hunch.
As always, I welcome feedback from (and this is a very low bar!) more competent Latinists than myself. Me, I'm just some guy with a Bible and a dictionary.