Hiranyakeshigrihyasutra 1.3.10.3

1.3.10.3

goṣṭhe vāvacchādya saṃpariśritya purodayamādityasya praviśati \ atra sarvaṃ kriyate \ nainametadaharādityobhitapatītyekeṣām \ snātānāṃ vā eṣa tejasā tapati ya eṣa tapati tasmātsnātakasya mukhaṃ rephāyatīva \3\

Or (instead of performing these rites in the neighbourhood of water) they make an enclosure in a cow-stable and cover it (from all sides); that (the student) enters before sunrise, and in that (enclosure) the whole (ceremony) is performed. 'On that day the sun does not shine upon him,' some say. 'For he who shines (i.e. the sun), shines by the splendour of those who have taken the bath. Therefore the face of a Snâtaka is, as it were, resplendent (?).'