Samyutta Nikaya. I,220

When in the forest amongst the roots of the trees

Or in the empty places,
Just call to mind the Buddha
And no fear of trembling will arise.

If you cannot think of the Buddha,
This best, this highest, this finest of beings,
Then call to mind the Dhamma,
The well taught guide.

If you cannot think of the Dhamma,
The well taught guide,
Then think of the Sangha,
That incomparable source of goodness in the world.