From Parmenides, the earliest source in the concept of timelessness:

. . . One path only
is left for us to speak of, namely, that It is. In this path are very many tokens
that what is is uncreated and indestructible;
for it is complete,[9] immovable, and without end.
Nor was it ever, nor will it be; for now it is, all at once,
a continuous one. For what kind of origin for it wilt thou look for?
In what way and from what source could it have drawn its increase? I shall not let thee say nor think
that it came from what is not; for it can neither be thought nor uttered
that anything is not. And, if it came from nothing, what need
could have made it arise later rather than sooner?
Therefore must it either be altogether or be not at all.
Nor will the force of truth suffer aught to arise
besides itself from that which is not. Wherefore,
justice doth not loose her fetters and let anything come into being or pass away,
but holds it fast. Our judgment thereon depends on this:
“Is it or is it not?” Surely it is adjudged, as it needs must be,
that we are to set aside the one way as unthinkable and nameless (for it is no true way),
and that the other path is real and true.
How, then, can what is be going to be in the future? Or how could it come into being?
If it came into being, it is not; nor is it if it is going to be in the future.
Thus is becoming extinguished and passing away not to be heard of.
Nor is it divisible, since it is all alike,
and there is no more[10] of it in one place than in another, to hinder it from holding together,
nor less of it, but everything is full of what is.
Wherefore it is wholly continuous; for what is, is in contact with what is.
Moreover, it is immovable in the bonds of mighty chains,
without beginning and without end; since coming into being and passing away
have been driven afar, and true belief has cast them away.
It is the same, and it rests in the self-same place, abiding in itself.
And thus it remaineth constant in its place; for hard necessity
keeps it in the bonds of the limit that holds it fast on every side.
Wherefore it is not permitted to what is to be infinite;
for it is in need of nothing; while, if it were infinite, it would stand in need of everything.[11]
The thing that can be thought and that for the sake of which the thought exists is the same;[12]
for you cannot find thought without something that is, as to which it is uttered.[13]
And there is not, and never shall be,
anything besides what is, since fate has chained it
so as to be whole and immovable. Wherefore all these things are but names
which mortals have given, believing them to be true—
coming into being and passing away, being and not being,
change of place and alteration of bright colour.
Since, then, it has a furthest limit, it is complete
on every side, like the mass of a rounded sphere,
equally poised from the centre in every direction; for it cannot be greater
or smaller in one place than in another.
For there is no nothing that could keep it from reaching
out equally, nor can aught that is be
more here and less there than what is, since it is all inviolable.
For the point from which it is equal in every direction tends equally to the limits