While the Buddha was staying at Savatthi, a band of monks, having received subjects of meditation from the master, proceeded to a forest to spend the rainy season (vassana). The tree deities inhabiting this forest were worried by their arrival, as they had to descend from tree abodes and dwell on the ground. They hoped, however, the monks would leave soon; but finding that the monks would stay the vassana period of three months, harassed them in diverse ways, during the night with the intention of scaring them away.

Living under such conditions being impossible, the monks went to the Master and informed him of their difficulties. Thereon the Buddha instructed them in the Metta sutta and advised their return equipped with this sutta for their protection.

The monks went back to the forest, and practicing the instruction conveyed, permeated the whole atmosphere with their radiant thoughts of metta or loving-kindness. The deities so affected by this power of love, henceforth allowed them to meditate in peace.

Karaniya Metta Sutta Sn 1.8 PTS: Sn 143-152 (Thanissaro Bhikkhu translation)

This is to be done by one skilled in aims
who wants to break through to the state of peace:
Be capable, upright, & straightforward,
easy to instruct, gentle, & not conceited,
content & easy to support,
with few duties, living lightly,
with peaceful faculties, masterful,
modest, & no greed for supporters.

Do not do the slightest thing
that the wise would later censure.

Think: Happy, at rest,
may all beings be happy at heart.
Whatever beings there may be,
weak or strong, without exception,
long, large,
middling, short,
subtle, blatant,
seen & unseen,
near & far,
born & seeking birth:
May all beings be happy at heart.

Let no one deceive another
or despise anyone anywhere,
or through anger or irritation
wish for another to suffer.

As a mother would risk her life
to protect her child, her only child,
even so should one cultivate a limitless heart
with regard to all beings.
With good will for the entire cosmos,
cultivate a limitless heart:
Above, below, & all around,
unobstructed, without enmity or hate.
Whether standing, walking,
sitting, or lying down,
as long as one is alert,
one should be resolved on this mindfulness.
This is called a sublime abiding
here & now.

Not taken with views,
but virtuous & consummate in vision,
having subdued desire for sensual pleasures,
one never again
will lie in the womb.