Is 1 Thessalonians 4.15-18 a Pauline quote of Jesus? Let's look at the suspect passages.
1 Thess 4.15-18
15 Spoken in the word of the Lord, we tell you that we who are still alive, who are left until the coming of the Lord, will certainly not precede those who have fallen asleep.
16 For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first.
17 After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever.
18 Therefore encourage one another with these words.
This is comparable to Mark 8.34-9.1, where Jesus mentions to a crowd that those who are standing there with him will “not taste death until they see the kingdom of god has come with power”:
34 Then he called the crowd to him along with his disciples and said: “Whoever wants to be my disciple must deny themselves and take up their cross and follow me.
35 For whoever wants to save their soul will lose it, but whoever loses their soul for me and for the good news will save it.
36 What good is it for someone to gain the whole world, yet forfeit their soul?
37 Or what can anyone give in exchange for their soul?
38 If anyone is ashamed of me and my words in this adulterous and sinful generation, the Son of Man will be ashamed of them when he comes in his Father’s glory with the holy angels.”
1 And he said to them, “Truly I tell you, some who are standing here will not taste death before they see that the kingdom of God has come with power.”
Jesus also repeats the same sentiment at his trial:
60 Then the high priest stood up before them and asked Jesus, “Are you not going to answer? What is this testimony that these men are bringing against you?”
61 But Jesus remained silent and gave no answer. Again the high priest asked him, “Are you the Messiah, the Son of the Blessed One?”
62 “I am,” said Jesus. “And you will see the Son of Man sitting at the right hand of the Mighty One and coming on the clouds of heaven.”
This implies that the high priest who tried Jesus should have seen the Son of Man descending from heaven during his lifetime.
So was Paul quoting Jesus? Based on these facts alone it would seem pretty obvious. But then there's my previous post
that shows Paul's inconsistent use of the non-titular “lord” in his letters. Or, his letters are quite consistent and that Paul thought that Jesus was present at the Exodus (1 Cor 10.9). Complicating things, the phrase “[by the] word of the lord” (λογω κυριου) that Paul uses in 1 Thess 4.14 is a somewhat standard phrase from the LXX (other times being rendered as ρημα κυριου), especially from the Nevi'im:
1 Kings 13.1
…εξ ιουδα παρεγενετο εν λογω κυριου
…by the word of the Lord [a man] came from Judah
και επεκαλεσεν προς το θυσιαστηριον εν λογω κυριου
And by the word of the Lord he cried out to the altar
ακουσατε λογον κυριου αρχοντες σοδομων
Hear the word of the Lord rulers of Sodom
…και λογος κυριου εξ ιερουσαλημ
…and the word of the Lord from Jerusalem
και εγενετο λογος κυριου προς με λεγων…
And the word of the Lord came to me saying…
ακουσατε λογον κυριου πασα η ιουδαια
Hear the word of the Lord all in Judah
και εγενετο μετα τας επτα ημερας λογος κυριου προς με λεγων
And after seven days the word of the Lord came to me saying
και εγενετο λογος κυριου προς με λεγων
And the word of the Lord came to me saying
λογος κυριου ος εγενηθη προς ωσηε…
The wordo f the Lord came to Hosea…
ακουσατε λογον κυριου υιοι ισραηλ…
Hear the word of the Lord children of Israel…
λογος κυριου ος εγενηθη προς ιωηλ τον του βαθουηλ
[The] word of the Lord came to Joel the son of [Pe]thuel
και νυν ακουε λογον κυριου…
And now hear the word of the Lord…
και εγενετο λογος κυριου προς ιωναν τον του αμαθι λεγων
And the word of the Lord came to Jonah the son of Amatti saying
και εγενετο λογος κυριου προς ιωναν εκ δευτερου λεγων
And the word of the Lord came to Jonah a second time saying
και εγενετο λογος κυριου προς μιχαιαν…
And the word of the Lord came to Micah…
λογος κυριου ος εγενηθη προς σοφονιαν…
[The] word of the Lord came to [Ze]ph[a]niah…
ουαι οι κατοικουντες το σχοινισμα της θαλασσης παροικοι κρητων λογος κυριου εφ' υμας χανααν γη αλλοφυλων
Woe to you who live by the sea, you Kerethite people; the word of the Lord is against you, Canaan, land of the Philistines.
…and so on throughout the books of the Prophets (and some Ketuvim like Daniel). So by itself, saying [by the] word of the Lord does not necessitate a saying of the Jesus of the gospel narratives. But I do think that Paul thought that Jesus “said” this, just as Paul probably thought that Jesus was the one giving inspiration to the Prophets.
What's interesting is that Paul doesn't number “the Lord's” παρουσια (arrival). He doesn't mention a “second arrival” (or second coming), which would be something like δευτερο παρουσια, but only “arrival”. Paul also explicitly states that the Lord will come down from heaven. This is consistent (I guess) with Paul thinking that the Lord was present at the Exodus and killed some Hebrews with snakes. Someone who was equal to (ειναι ισα θεω) and in the form of (μορφη θεου) Yahweh (Phil 2.6) and through whom the world was created and sustained by would most certainly have existed during the Exodus.
Many scholars believe that the part of Philipians that I quoted is a hymn that precedes Paul, thus there would have been other Christians – prior to Paul – who thought that Jesus was in the form of and equal to Yahweh. And if Jesus created and sustained the world, then it would make sense that he was present at the Exodus.
So is Paul quoting Jesus? I think he is, but he's not quoting any earthly Jesus. Paul probably got this “word of the Lord” by revelation.