This is one of my favourite suttas:

SN 1.10 Arañña Sutta: A Face So Calm


Those who abide in the forest,
Peaceful, living the holy life;
Those who eat but a single meal;
— why is it their face is so calm?

[The Buddha:]

They do not grieve over the past,
Nor do they yearn for the future;
They live only in the present
— That is why their face is so calm.

It's from yearning for the future,
And from grieving over the past;
This is how fools become withered
— Like a fresh reed that's been hacked down.