SB 3.12.19

tapasaiva paraṁ jyotir
 bhagavantam adhokṣajam
sarva-bhūta-guhāvāsam
 añjasā vindate pumān
Word for word: 
tapasā — by penance; eva — only; param — the supreme; jyotiḥ — light; bhagavantam — unto the Personality of Godhead; adhokṣajam — He who is beyond the approach of the senses; sarva-bhūta-guhā-āvāsam — residing in the heart of all living entities; añjasā — completely; vindate — can know; pumān — a person.
Translation: 
By penance only can one even approach the Personality of Godhead, who is within the heart of every living entity and at the same time beyond the reach of all senses.
Purport: 

Rudra was advised by Brahmā to perform penance as an example to his sons and followers that penance is necessary for attaining the favor of the Supreme Personality of Godhead. In Bhagavad-gītā it is said that the common mass of people follow the path shown by an authority. Thus Brahmā, disgusted with the Rudra generations and afraid of being devoured by the increase of population, asked Rudra to stop producing such an unwanted generation and take to penance for attaining the favor of the Supreme Lord. Therefore we find in pictures that Rudra is always sitting in meditation for the attainment of the favor of the Lord. Indirectly, the sons and followers of Rudra are advised to stop the business of annihilation, following the Rudra principle while the peaceful creation of Brahmā is going on.