Prashna Upanishad 77.3

77.3

sa punarbhavaṃ śarīrāntaraṃ pratipadyate /
katham?sahendriyairmanasi sampadyamānaiḥ praviśadbhirvāgādibhiḥ //9//

That which is tejaḥ ha vai, well known as (common)footnote{As distinguished from its special manifestation as the sun.} luminosity, outside; is udānaḥ, Udāna, in the body; that is to say, it favours the vital function, called Udāna, by its light. Since the agency (viz Udāna), that causes one's leaving the body, is of the nature of luminosity, and (while staying in the body) it is favoured by external light tasmāt, therefore: when an ordinary man upaśāntatejāḥ, gets his natural light extinguished; then it is to be understood that his life is exhausted and he is about to die. Saḥ, he, attains; punarbhavam, another body, (rebirth). How? Saha indriyaiḥ, together with the organs, counting from speech: sampadyamānaiḥ manasi, entering into the mind.