There is special providence in the fall of a sparrow
Horatio:
If your mind dislike any thing, obey it. I will forestall their
repair hither, and say you are not fit.
Hamlet:
Not a whit, we defy augury. There is special providence in
the fall of a sparrow. If it be now, 'tis not to come; if it be not to
come, it will be now; if it be not now, yet it will come—the
readiness is all. Since no man, of aught he leaves, knows what is't
to leave betimes, let be.
Hamlet's stepfather, King Claudius, has arranged a fencing match between the prince and Laertes. Laertes happens to be the son of Polonius (whom Hamlet has slain) and the brother of Ophelia (who has gone mad and committed suicide as a result of Hamlet's actions). Hamlet and his friend Horatio well know that the king desperately wants the prince out of the way, and that Laertes is looking for revenge; the fencing match doesn't promise to be an entirely playful affair.
Hamlet has agreed to it nonetheless, and refuses Horatio's offer to excuse him if he thinks better of things. "We defy augury"—that is, omens mean nothing to him. Hamlet will deliver himself over to his fate, because he finally realizes that it is out of his control. Before, he would have thought too precisely on the event, weighed its implications, and sought into its causes. Now, he is of the opinion that "there's special providence in the fall of a sparrow," and therefore a guiding hand behind his own fall, whenever it comes, now or in the future. Here, Hamlet echoes the Gospel according to St. Matthew, chapter 10: "And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell./ Are not two sparrows sold for a farthing? and one of them shall not fall on the ground without your Father" (King James version).