Isaiah 8:12
11 Indeed this is what the Lord told me quite forcefully. He warned me not to act like these people: 12 “Do not say, ‘Conspiracy,’ every time these people say the word. Don’t be afraid of what scares them; don’t be terrified. 13 You must recognize the authority of the Lord of Heaven’s Armies. He is the one you must respect; he is the one you must fear. 14 He will become a sanctuary, but a stone that makes a person trip and a rock that makes one stumble—to the two houses of Israel. He will become a trap and a snare to the residents of Jerusalem. 15 Many will stumble over the stone and the rock, and will fall and be seriously injured, and will be ensnared and captured.” 16 Tie up the testimony32, seal the official record of God’s instructions, and give it to my followers.
1 Peter 3:14
13 For who is going to harm you if you are devoted to what is good? 14 But in fact, if you happen to suffer for doing what is right, you are blessed. But do not be terrified of them or be shaken. 15 But set Christ apart as Lord in your hearts and always be ready to give an answer to anyone who asks about the hope you possess. 16 Yet do it with courtesy and respect, keeping a good conscience, so that those who slander your good conduct in Christ may be put to shame when they accuse you. 17 For it is better to suffer for doing good, if God wills it, than for doing evil.
Notes and References
"... Though the author does not introduce the words of Isaiah 8:12-13 with an introductory formula, it is clear that it is an explicit quotation, for: (1) An abrupt and somewhat unusual phrase is in exact agreement with the LXX; (2) The two [key] verbs are identical to the LXX; (3) There follows a contrasting command to reverence the Lord. The author is clearly wishing to direct his readers to the LXX of Isaiah 8:12-13. However, having ascertained the author’s intention to quote, it is now necessary to explore the differences between the two texts ..."
van Rensburg, Fika J. Isaiah in 1 Peter 3:13-17 Applying Intertextuality to the Study of the Old Testament in the New (pp. 275-286) Scriptura 80, 2002