Which Old Testament prophet does Paul quote in Romans 9:27 about Israel?

Les explains that in Romans 9:27 Paul goes to yet another portion of the Old Testament, specifically to Isaiah. The quotation says, "And even Isaiah crieth concerning Israel, '...though the number of the children of Israel be as the sand of the sea, a remnant shall be saved.'" Les stresses what he calls the doctrine of the remnant: all through Scripture God has never enjoyed the majority, but always a small remnant. He illustrates with the flood, where only eight remained true to God, and with Israel, where the Word would light on Jacob, the whole nation, but only a few responded and believed. Les says it is the same way here from Isaiah: God has always had His remnant, even within the nation of Israel.