From the Bible:

Chapter 4 of the Gospel of St. John

The hour cometh, and now is, when the true worshippers shall worship the Father in spirit and in truth: for the Father seeketh such to worship Him.

God is a Spirit: and they that worship him must worship him in spirit and in truth.

From the Bhagavad Gita, sixth Chapter:

Sequestered should he sit,
Steadfastly meditating, solitary,
His thoughts controlled, his passions laid away,
From every craving for possession freed.