Khandogya-Upanishad: Fourth Prapathaka, Thirteenth Khanda
1. Then the Ahavanaya fire taught him: “Breath, ether, heaven, and lightning (these are my forms). The person that is seen in the lightning, I am he, I am he indeed.
2. “He who knowing this meditates on him, destroys sin, obtains the world (of Agni Ahavaniya), reaches his full age, and lives long; his descendants do not perish. We guard him in this world and in the other, whosoever knowing this meditates on him.”