Variantly: "And I shall see the blood": I shall see the "blood" of the binding of Isaac, as it is written (Genesis 22:14) "And Abraham called the name of the place 'The L–rd will see.'" And elsewhere it is written (I Chronicles 21:15) "But as he was about to destroy, the L–rd saw and He repented Himself of the evil." What did He see? The blood of the binding of Isaac.
What Did God See at That Moment of Impending Destruction
Mekhilta Tractate Pischa 7:23