Clement of Alexandria (AD 150-215) believed the Epistle to the Hebrews was written by Paul in the Hebrew language, then translated into Greek by Luke. This is wrong:
Decisive evidence against this can be found in the extensive use of the Septuagint throughout the book (see the article by Kenneth J. Thomas in NTS, 1965), with the author using the Greek version of the OT. Also there is the pun in Hebrews 5:8 (ἔμαθεν ἀφ’ ὧν ἔπαθεν), and similar μαθεῖν/παθεῖν wordplay can be found in Aristotle, Herodotus, Aeschylus, and Plato (see Heinrich Dörrie’s article in Akademie der Wissenschaften und der Literatur, 1956, which was reviewed in TCR, 1958).
Thomas’ article goes through the allusions and quotations one by one, so I’ll mention an example. Hebrews 13:5 quotes God as saying that he would never leave or forsake you (οὐ μή σε ἀνῶ, οὐδ’ οὐ μή σε ἐγκαταλίπω), and this quotes לא ארפך ולא אעזבך found in Joshua 1:5 (cf. לא אעזבך in Genesis 28:15) which also occurs in the third person (לא ירפך ולא יעזבך) in Deuteronomy 31:6, 8 and 1 Chronicl?
es 28:20.

οὐ μή σε ἐγκαταλίπω (Genesis 28:15 LXX)

οὐκ ἐγκαταλείψω σε οὐδὲ ὑπερόψομαί σε (Joshua 1:5 LXX)

οὐ μή σε ἀνῇ οὔτε μή σε ἐγκαταλίπῃ (Deuteronomy 31:6 LXX)

οὐκ ἀνήσει σε οὐδὲ μὴ ἐγκαταλίπῃ σε (Deuteronomy 31:8 LXX)

οὐκ ἀνήσει σε καὶ οὐ μή σε ἐγκαταλίπῃ (1 Chronicles 28:20 LXX)

οὐ μή σε ἀνῶ, οὐδ’ οὐ μή σε ἐγκαταλίπω (Hebrews 13:5)

These examples show variations in word choice (ὑπεροράω “overlook” vs. ἐγκαταλείπω “forsake”), use of μή with οὐ, and word order with the placement of σε before or after the verb. The wording in Hebrews 13:5 closely matches Deuteronomy 31:6 LXX and modifies it by adding another οὐ in the second clause and changing the verbs from third person to first person. There was a much wider set of options than found in just these examples from the LXX. Field show that in Deuteronomy 31:6, Aquila had παρήσει (a form of παρίημι “disregard”) in place of the LXX’s ἀνῇ (a form of ἀνίημι “desert, leave”) for Hebrew ירפך (which shares with both παρίημι and ἀνίημι a connotation of loosening up or making something go slack). What is really interesting about this example is that we have a pre-Hexaplaric attestation of the verse in Philo of Alexandria (De Confusione Linguarum, 166) who gives exactly the same form that is found in Hebrews 13:5: “And therefore the merciful God has delivered an oracle full of loving-kindness which has a message of good hope to the lovers of discipline. It is to this effect: ‘I will not let you go nor will I abandon you’ (οὐ μή σε ἀνῶ, οὐδ᾿ οὐ μή σε ἐγκαταλίπω)”. This suggests that the author of Hebrews was either dependent on an Alexandrian form of the Greek OT or perhaps on Philo himself (who of course wrote in Greek).