...and is what an incipit does.
An incipit consists of the first few words of a book (or the first few notes of a piece of music).
Nowadays we usually file books by their titles, but titles weren't invented until several thousand years after books came into being.
The clay tablets of Sumer, for instance, were catalogued under titles which included Where are the sheep and And with you I did not.
If that seems a bit silly, then consider: it's exactly the same system as is used to make up a title for a WORD document, isn't it.
Nowadays it's poems that most often are known by their incipits. This is often because the poems don't have proper titles, being labelled Sonnet, for instance, in which case we'll often refer to a poem as something like: How do I love thee? Let me count the ways. rather than Sonnet from the Portugese No 43. Music has the same problem. With a million pieces called Allegro, an album of pieces will often begin with a page of first phrases written out so you can find the piece you want.
Hymns tend to be listed by their incipits, too, which is fine unless they're best known for their chorus or refrain. It can take a long time to find O Still Small Voice of Calm* in a hymn book index, or To be a Pilgrim** or that one, you know, the Mr Bean one, which goes Hallelu-u-u-u-yah!***
Thing to use today: an incipit. This word is Latin, and means here begins.
* Dear Lord and Father of Mankind.
**He who would valiant be.
***All Creatures of Our God and King.