Jamieson, Fausset and Brown on Genesis 22:4

4. on the third day Abraham lifted up his eyes, &c.--Leaving the servants at the foot [Ge 22:5], the father and son ascended the hill, the one bearing the knife, and the other the wood for consuming the sacrifice [Ge 22:6]. But there was no victim; and to the question so naturally put by Isaac [Ge 22:7], Abraham contented himself by replying, "My son, God will provide himself a lamb for a burnt offering." It has been supposed that the design of this extraordinary transaction was to show him, by action instead of words, the way in which all the families of the earth should be blessed; and that in his answer to Isaac, he anticipated some substitution. It is more likely that his words were spoken evasively to his son in ignorance of the issue, yet in unbounded confidence that that son, though sacrificed, would, in some miraculous way, be restored (He 11:19).