Genesis 8:20

Noah builded an altar The first altar that we read of; but not the first which was built; for the sacrifices which were offered before, Genesis 4:3, presuppose an altar or altars. And it ought to be well observed, that the silence of Scripture concerning any thing is not sufficient evidence that it was not done; to remember which will greatly assist us in understanding many passages of the sacred oracles. Here we see, that the first thing that he did after his wonderful preservation was to pay this debt of gratitude so justly due to that God who had so wonderfully preserved him. Hitherto he had done nothing without particular instructions and commands from God: but altars and sacrifices being already of divine institution, he did not stay for a particular command thus to express his thankfulness. And he offered on the altar, of every clean beast, and of every fowl One, the odd seventh that we read of, Genesis 7:2.