The answer is: Walt Whitman’s poem “I Hear America Singing.”
An allusion is something said intended to make somebody think of a particular person, thing or event; or a short and indirect reference.
By making use of allusion to Whitman's poem in "I, Too, Sing America" Hughes could relate to American cultural heritage. For example, he uses Whitman's thought that all Americans are important members of society, and claims for social change and equal rights for African Americans.
