Line 223: Shylock: "A Daniel come to judgement! yea, a Daniel."
Line 333: Gratiano: "A second Daniel! A Daniel, Jew!" (sarcastic)
Line 339: Gratiano: "A Daniel, still say I, a second Daniel!"
The allusion is clearly to the story of Susanna and the elders, one of the addtiions to the book of Daniel.
Actually, I meant a link to the books of the Bible. And you might want to read the books by Pearce on Shakespeare.