It is specifically recorded in Rev 7:1-8; 14:1-5.
The 144,000 in Rev 7:1-8 are different then the 144,00 in Rev 14:1-5. One count is literal and the other symbolic.in it's numbering as being many.
Rev 7:1-8 These servants are the generational 144,000 from the twelve tribes of Israel, except for the tribe of Dan and Ephraim as they fell to idol worship and allowed God to be removed from them, Judges 18:30; 1 Kings12:25-33. To keep it twelve tribes since Dan and Ephraim are not mentioned in vs. 4-8 the tribe of Joseph appears twice, once under his own name and once under the name of his son Issachar/Manasseh. Judah is mentioned first as Messiah was to come from the lineage of David.
This is the importance of the 144,000 of the house of Israel and their generations unto the end of days. There were those of the old covenant that were faithful to God in keeping all his commandments and repented of their idol worshipping. Their generations who have also come back to Gods grace that are still alive during the seven trumpet sounds will continue to preach to the Jews as it was with the twelve disciples in Matthew 10:6. The generations of the 144,000 that are still alive have the seal of God on them which are the seven Spirits of God as they will have to endure much persecution until the end of days before the return of Jesus.
Rev 14:1-5 The Lamb represents Jesus who stands on the symbolic mount Sion in the heavenly Jerusalem meaning where God, Jesus and the holy angels dwell which is called the heaven of heavens, Hebrews 12:22-24; Deuteronomy 10:14; 1 Kings 8:27; Psalms 115:16. The hundred and forty four thousand that John sees standing with Jesus is not a literal numbering, but symbolic of those of Gods established covenant with faithful Abraham up through a thousand generations, Deuteronomy 7:9; Galatians 3:6, 7; 3:28, 29. These are they of great faith that have never allowed them selves to fall to temptations of the world. To the Jew first and then the Gentile who have gone on before us as together with them all who are alive or have died before the Lords return makes up the complete body of Christ, Ephesians 5:25-27.
God breathed life into us, Genesis 2:7, and when this flesh body dies that spirit of life that was breathed in us will be the only thing that will go back to God, Ecclesiastics 12:7. This fleshly body will return to the dust of the ground from where it came from until those who have died that are in Christ are resurrected first as we who are still alive at this time will be gathered with them in the clouds to meet Jesus in the air, John 3:1-6; 1 Corinthians 15:52; 1 Thessalonians 4:13-18.