The line you quoted is actually:
> "Let us go then, you and I,
> When the evening is spread out against the sky
> Like a patient etherised upon a table;
> Let us go, through certain half-deserted streets,
> The muttering retreats
> Of restless nights in one-night cheap hotels
> And sawdust restaurants with oyster-shells:
> Streets that follow like a tedious argument
> Of insidious intent
> To lead you to an overwhelming question...
> Oh, do not ask, “What is it?”
> Let us go and make our visit.
The bolded lines are the ones you were likely thinking of. While they don't contain the exact words "belt doth span sky," the imagery is quite similar, especially with "spread out against the sky" and the comparison to a table.
It's possible you misremembered the lines, but this poem is a very famous one, and it's the most likely source of the imagery you described.