Isaiah 7:14 (BSB)

Therefore the Lord Himself will give you a sign: Behold, the virgin will be with child and give birth to a son, and will call Him Immanuel.

From Isaiah 7. Also in the ESV.

Commentary on Isaiah 7:14

• John Calvin (Reformed), Calvin's Commentaries on Isaiah 7:14: 14. Therefore the Lord himself shall give you a sign. Ahaz had already refused the sign which the Lord offered to him, when the Prophet remonstrated against his rebellion and ingratitude; yet the Prophet declares that this will not prevent God from giving the sign which he had promised and appointed for the Jews. But what sign? Behold, a virgin shall conceive.
• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on Isaiah 7:10-16: Here, I. God, by the prophet, makes a gracious offer to Ahaz, to confirm the foregoing predictions, and his faith in them, by such sign or miracle as he should choose (Isa 7:10, Isa 7:11): Ask thee a sign of the Lord thy God; See here the divine faithfulness and veracity. God tells us nothing but what he is able and ready to prove.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on Isaiah 7:14: Therefore the Lord himself shall give you a sign,.... Whether they would ask one or not; a sign both in heaven and earth, namely, the promised Messiah; who being the Lord from heaven, would take flesh of a virgin on earth; and who as man, being buried in the heart of the earth, would be raised from thence, and ascend up into heaven; and whose...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on Isaiah 7:14: himself--since thou wilt not ask a sign, nay, rejectest the offer of one. you--for the sake of the house of believing "David" (God remembering His everlasting covenant with David), not for unbelieving Ahaz' sake. Behold--arresting attention to the extraordinary prophecy. virgin--from a root, "to lie hid," virgins being closely kept from men's gaze in their parents' custody in the East.