In the beginning, the great Vishnu desirous of creating the
whole world became threefold: creator, preserver, and destroyer.
In order to produce this world, the Supreme Spirit emanated
from the right side of his body himself as Brahma.
Then in order to preserve the universe, he produced from the
left side of his body Vishnu
In order to destroy the world, he produced form the middle
of his body the eternal Shiva.
Some worship Brahma, some Vishnu, and other Shiva; but
Vishnu, one yet threefold, creates, preserves, and destroy; therefore let the
pious make no difference between the three.
Padma Purana
Padma Purana