incipit

noun

the introductory words or opening phrases in the text of a medieval manuscript or an early printed book.
Music. the first words of a chanted liturgical text, as that of a Gregorian chant or certain medieval motets.

here begins: used as an introductory word at the beginning of some medieval manuscripts

opening word of a Latin book or manuscript, Latin, literally “(here) begins,” third person singular present indicative of incipere (see incipient).