Tafsir of Al-Mu'minoon 23:83

"(We have been promised this) — meaning resurrection — (before) — this is connected to the verb in terms of its attribution to both the subject and the coordinate, which is the apparent meaning. This is valid regarding them because the prophets who informed of resurrection were informing of it for everyone who dies.

It is also possible that it is connected to the verb in terms of its attribution to their forefathers, not to them; that is, 'our forefathers were promised before.' Or, it may be connected to an omitted element serving as a circumstantial qualifier (hal) for our forefathers, meaning 'existing before.'

(This is nothing) — meaning, this is not— (but myths of the ancients) — that is, their lies which they inscribed. Asatir is the plural of usturah, similar to uhduthah (a tale) and u’jubah (a wonder). Al-Mubarrad and a group of scholars held this view. It has been said that it is the plural of astar, which is the plural of satr (a line/writing), like faras and afras. The first view, as Al-Zamakhshari stated, is more appropriate because the pluralization of a singular noun is more fitting and follows standard grammatical rules, and because the structure uf’ulah typically denotes that which is used for amusement. Thus, it is as if it were said: 'written matter that has no substance to it.'"