Srimad Bhagavatam Canto 08, Chapter 07, Text 29

SB 8.7.29

mukhani pañcopanisadas tavesa
 yais trimsad-astottara-mantra-vargah
yat tac chivakhyam paramatma-tattvam
 deva svayam-jyotir avasthitis te
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
O lord, the five important Vedic mantras are represented by your five faces, from which the thirty-eight most celebrated Vedic mantras have been generated. Your Lordship, being celebrated as Lord Siva, is self-illuminated. You are directly situated as the supreme truth, known as Paramatma.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
The five mantras mentioned in this connection are as follows: (1) Purusa, (2) Aghora, (3) Sadyojata, (4) Vamadeva, and (5) Isana. These five mantras are within the category of thirty-eight special Vedic mantras chanted by Lord Siva, who is therefore celebrated as Siva or Mahadeva. Another reason why Lord Siva is called Siva, which means “all-auspicious,” is that he is self-illuminated, exactly like Lord Visnu, who is the Paramatma. Because Lord Siva is directly an incarnation of Lord Visnu, he is situated as Lord Visnu’s direct representative. This fact is corroborated by a Vedic mantra: patim visvasyatmesvaram sasvatam. sivam acyutam. The Supersoul is called by many names, of which Mahesvara, Siva and Acyuta are especially mentioned.
Srimad Bhagavatam Canto 08, Chapter 07, Text 28
Srimad Bhagavatam Canto 08, Chapter 07, Text 30