Psalm 94:7-8

As long as were on the Liturgy side of things: at my favorite high school, back when Father M was around, we'd say "If today you hear his voice / O harden not your hearts", I believe as the psalm antiphon, but possibly as the alleluia verse; I wasn't as attentive to the mass back then. At any rate, in my study of the Liturgy of the Hours, Latin style, I encountered that very verse in Psalm 94, used in the Invitiatory!

Utinam hodie vocem eius audiatis:
Nolite obdurare corda vestra

As seems to almost always be the case, I'm more fond of the Latin rendition. "Utinam" means something to the effect of "if only", "would that" or "oh, that"; it is an adverb of longing. "Nolite" comes from the verb "nolle", and is in the imperitive form. It's the subtulties that did me in: what an opportunity you have to hear his voice today! Don't you dare harden your heart.