III. /^Aandogya-Upanishad VIII. 6, 5 :

'When he departs from this body he mounts up- wards by those very rays (the rays of the sun which enter the arteries of the body), or he is removed while saying Om 1. And quickly as he sends off his mind (as quick as thought), he goes to the sun. For the sun is the door of the world (lokadvaram), an entrance for the knowing, a bar to the ignorant.'