Which literary device do you find in the expression:“We were the first that ever burstInto that silent sea”?

The line taken from Coleridge’s famous poem ‘The Rime of the Ancient Mariner’ is a nice example of alliteration.
Alliteration refers to repetition of a consonant in two or more words:
“We were the first that ever burst
Into that silent sea”

Source: An ABC of English Literature by Dr. M. Mofizar Rahman