Translation of the verse 23 from Surah An-Naba’ : Number of verses 40 - - page 582 - Part 30.
They will abide therein for ages,
In which they will remain for ages [unending].
(78:23) therein they shall abide for ages, *15
*15) The word ahqab as used in the original means successive periods of long time appearing continuously one after the other. From this word some people have tried to argue that there will be eternity in the life of Paradise but no eternity. in the life of Hell. For however long these ages may be, they will not be endless but will come to an end at some time. But this argument is wrong for two reasons. First, that lexically, the word haqab (sing. of ahqab) itself contains the meaning that one haqab should be closely followed by another haqab; therefore, ahqab will necessarily be used only for such periods of time as continue to appear successively one after the other and there should be no period which is not followed by another period. Second, that as a rule it is wrong to put a meaning on a verse of the Qur'an pertaining to a particular theme which clashes with other statements of the Qur'an pertaining to the same theme. At 34 places in the Qur'an the word khulud (eternity) has been used concerning the dwellers of Hell. At three places not only the word khulud has been used but the word abad an (for ever and ever) also has been added to it; and at one place it has been clearly stated: "They will wish to get out of Hell but shall not be able to come out of it and theirs shall be an everlasting torment." (AI-Ma'idah: 37). At another place it has been said: "Therein they shall abide for ever, as long as the earth and the heavens shall last, unless your Lord ordains otherwise." And the same thing has been said about the dwellers of Paradise too:" "They shall dwell in Paradise for ever, as long as the earth and the heavens shall last, unless your Lord wills something else." (Hud: 107-108). After these explanations, how can one argue, on the basis of the word abqab, that the stay of the rebels of God in Hell will not be eternal, but it will come to an end at some stage in time?
Source : An-Naba’ Verse 23: In which they will remain for ages [unending].