Three passages carry most of the weight. In the Olivet Discourse, given according to Matthew, Mark and Luke on the Mount of Olives shortly before his arrest, Jesus predicts the destruction of the Jerusalem Temple and describes signs, wars, false prophets, cosmic disturbance, that will precede "the Son of Man coming in the clouds with power and great glory." He adds a time marker that has driven most of the later debate: "this generation will not pass away until all these things have taken place."
Paul's first letter to the Thessalonians, written around 50 CE and among the earliest surviving Christian documents, describes the Lord descending from heaven with a call and a trumpet, the dead in Christ rising first, and then, in Paul's own words, "we who are alive, who are left, will be caught up together with them in the clouds," phrasing that assumes he and his readers might themselves live to see it.
After the ascension narrative in Acts, two messengers tell the assembled disciples that "this Jesus, who has been taken up from you into heaven, will come in the same way as you saw him go," fixing the expectation to a literal, visible return. Revelation's later visions of Christ's coming draw on the same expectation but in a heavily symbolic apocalyptic register that later interpreters, across nearly every tradition, read far less literally than the other three.
Sources The New Testament, Gospel of MatthewAnonymous with The New Testament, 1 ThessaloniansPaul the Apostle, The New Testament, Acts of the ApostlesAnonymous