Laetetur cor quaerentium Dominum: quaerite Dominum, et confirmamini: quaerite faciem eius semper.
“The heart of those who seek the Lord shall rejoice. Seek the Lord and be strengthened, seek his face always.” (Ps 105(104):3–4)
Click here to listen to an audio of the chant, sung by Liborius Lumma, Innsbruck (Austria).
We had the same introit on the 4th Sunday OT, so here is the link where you can read my thoughts about revera Deum quaerere.