Arañña Sutta: The Wilderness (SN 1:10)

Standing to one side, a devata addressed the Blessed One with a verse:

Living in the wilderness,
staying peaceful, remaining chaste,
eating just one meal a day:
why are their faces
so bright & serene?

[The Buddha:]

They don't sorrow over the past,
don't long for the future.
They survive on the present.
That's why their faces
are bright & serene.
From longing for the future,
from sorrowing over the past,
fools wither away
like a green reed cut down.