What is the source of the allusion in the sentence below?
When he was eighteen, in what he always called his "Horatio Alger moment," my grandfather boarded a bus to New York City with twenty dollars in his pocket and a plan to start his own business.

 - literature
 - Shakespeare
The source of the allusion Horatio Alger is literature.
In the novels of nineteenth-century American author Horatio Alger, protagonists invariably rose from poverty to wealth through hard work and honesty.
The allusion Horatio Alger means rags-to-riches.