If I can understand the question correctly, you are asking for when was the concept of hell came into being in the bible.
I would suggest it came into being after the fall of satan and his followers.
Matthew 25:41 - King James Version
41 Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:
The scripture clearly states as everlasting fire was prepared for the devil and his angels and not for humans. The scriptures also mentions that it's not God's will that his creation (humans) have everlasting life look at John 3:16
Hell is for real and it's all over the Holy Bible, The word “hell” appears 54 times in the Holy Bible.
Reality of Hell:
- Matthew 8:12, 22:13, 24:51, 25:30, 13:28 There is darkness, weeping
and gashing of teeth
- 2 Samuel 22:6, Psalm 18:5 There are sorrows in hell
- Job 11:8 Hell is deep
- Psalm 86:13 Lowest hell
- Psalm 116:3 There is pain in hell
- James 3:6 Your tongue is set on fire in hell
- Isaiah 33:10-17 There is a devouring fire, and everlasting burning
- Isaiah 66:24 worms never die and fire not quenched
- Matthew 25:41 Everlasting Fire
- Revelation 14:9-12, 20:10 You are tormented with fire and brimstone
in God’s presence
- Revelation 21:8 It is a lake of fire and that is the second
A flood of false doctrine has lately broken in upon us. Men are beginning to tell us “that God is too merciful to punish souls for ever...that all mankind, however wicked and ungodly...will sooner or later be saved.” We are to embrace what is called “kinder theology,” and treat hell as a pagan fable... This question lies at the very foundation of the whole Gospel. The moral attributes of God, His justice, His holiness, His purity, are all involved in it. The Scripture has spoken plainly and fully on the subject of hell... If words mean anything, there is such a place as hell. If texts are to be interpreted fairly, there are those who will be cast into it... The same Bible which teaches that God in mercy and compassion sent Christ to die for sinners, does also teach that God hates sin, and must from His very nature punish all who cleave to sin or refuse the salvation He has provided. -- J.C. Ryle (1816-1900).
If you are looking for merely the oldest chronological reference, Evangelicals in particular tend to consider Job to be the book in the Bible that refers to the oldest time period post-Genesis. (This is due to a seeming lack of reference to Torah code) Job would thus be the "oldest" reference to hell.