The premise of this thread is very simple - we'll just reference quotations and possible allusions in the book of Revelation to other books and other stuff, or at least passages which happen to say the same thing. I am mostly going here by the marginal apparatus in Nestle-Aland's Novum Testamentum Graece, 27th edition (NA27). Here goes:
1:1a "The revelation of Jesus Christ, which God gave him to show to his servants the things that must soon come to pass." (Ἀποκάλυψις Ἰησοῦ Χριστοῦ ἣν ἔδωκεν αὐτῷ ὁ θεὸς δεῖξαι τοῖς δούλοις αὐτοῦ ἃ δεῖ γενέσθαι ἐν τάχει.)
(Amos 3:7) For the Lord God does nothing without revealing his secret to his servants the prophets.
(Daniel 2:28-47 Theodotion) “But there is a God in heaven revealing mysteries, and He has made known to king Nabuchodonosor what things must come to pass in the last days (ἃ δεῖ γενέσθαι ἐπ' ἐσχάτων τῶν ἡμερῶν).” ...] And the king answered and said to Daniel, “Of a truth your God is a God of gods and Lord of kings who reveals mysteries (καὶ ἀποκαλύπτων μυστήρια); for you have been able to reveal this mystery (ἀποκαλύψαι τὸ μυστήριον τοῦτο).”
1:1b-2 "And he made it known by sending his angel to his servant John, who testified to the word of God and to the testimony of Jesus Christ, to all that he saw." (καὶ ἐσήμανεν ἀποστείλας διὰ τοῦ ἀγγέλου αὐτοῦ τῷ δούλῷ αὐτοῦ Ἰωάννῃ, ὃς ἐμαρτύρησεν τὸν λόγον τοῦ θεοῦ καὶ τὴν μαρτυρίαν Ἰησοῦ Χριστοῦ ὅσα εἶδεν.)
(1 John 1:1-4) That which was from the beginning, which we have heard, which we have seen with our eyes, which we looked upon and have touched with our hands, concerning the word of life—the life was made manifest, and we have seen it, and testify to it and proclaim to you the eternal life, which was with the Father and was made manifest to us—that which we have seen and heard we proclaim also to you, so that you too may have fellowship with us; and indeed our fellowship is with the Father and with his Son Jesus Christ. And we are writing these things so that our joy may be complete.
1:3 "Blessed is he who reads aloud the words of this prophecy, and blessed are those who hear, and who keep what is written in it, for the time is near." (μακάριος ὁ ἀναγινώσκων καὶ οἱ ἀκούοντες τοὺς λόγους τῆς προφητείας καὶ τηροῦντες τὰ ἐν αὐτῇ γεγραμμένα, ὁ γὰρ καιρὸς ἐγγύς.)
(Psalm 1:1-2) Blessed is the man
who walks not in the counsel of the wicked,
nor stands in the way of sinners,
nor sits in the seat of scoffers;
but his delight is in the law of the LORD,
and on his law he meditates day and night. (Wisdom mode)
(Daniel 12:12-13) Blessed is he who waits and arrives at the 1,335 days. But go your way till the end. And you shall rest and shall stand in your allotted place at the end of the days. (Apocalyptic mode)
1:4 "John to the seven churches that are in Asia: grace to you and peace from him who is and who was and who is coming, and from the seven spirits who are before his throne," (Ἰωάννης ταῖς ἑπτὰ ἐκκλησίαις ταῖς ἐν τῇ Ἀσίᾳ· χάρις ὑμῖν καὶ εἰρήνη ἀπὸ ὁ ὢν καὶ ὁ ἦν καὶ ὁ ἐρχόμενος καὶ ἀπὸ τῶν ἑπτὰ πνεύματων ἃ ἐνώπιον τοῦ θρόνου αὐτοῦ)
(Exodus 3:13-14 LXX) And Moyses said to God, “Look, I shall come to the sons of Israel and shall say to them, ‘The God of your fathers has sent me to you’; they will ask me, ‘What is his name?’ What shall I say to them?” And God said to Moyses, “I am The One Who Is.” (ὁ ὢν) And he said, “Thus you shall say to the sons of Israel, ‘The One Who Is has sent me to you’.”
(Isaiah 41:4 LXX) Who has wrought and done these things?
The one calling her from the beginning of generations
has called her.
I, God, am first,
and for the things that are coming, I am.
(Plutarch, De Iside et Osiride, 9) Moreover, the temple of Minerva which is at Sais (whom they look upon as the same with Isis) had upon it this inscription: I am whatever was, or is, or will be; and my veil no mortal ever took up.
(Pausanias, *Description of Greece *10.12.10) The Peleiades are said to have been born still earlier than Phemonoe, and to have been the first women to chant these verses:—“Zeus was, Zeus is, Zeus shall be; O mighty Zeus. Earth sends up the harvest, therefore sing the praise of earth as Mother.”