The poetic device which the poet Emily Dickinson uses in her poem “Because I Could Not Stop For Death" in the line ' my labor and my leisure too' is alliteration.
Alliteration is a literary device in which words with the same consonant sound or syllable occur. It gives a poem a different and a unique rhythm. Alliteration is a special case of consonance when the consonant sound of the stressed syllable is repeated.
In the line, the sound 'm' occurs in the word “my” and the sound 'l' occurs in the words “labor” and “leisure.”