Shatapatha Brahmana 14.1.1.33

14.1.1.33

athaitadvā āyuretajjyotiḥ praviśati ya etamanu vā brūtebhakṣayati vā tasya vratacaryā nātapati pracādayeta nedetasmāttiro'sānīti nātapati niṣṭhīvennedetamabhiniṣṭhīvānīti nātapati prasrāvayeta nedetamabhiprasrāvayā iti yāvadvā eṣa ātapati tāvāneṣa nedetametairhinasānītyavajyotya rātrāvaśnīyāttadetadasya rūpaṃ kriyate ya eṣa tapati tadu hovācāsurirekaṃ ha vai devā vrataṃ caranti yatsatyam tasmādu satyameva vadet

And, verily, he who either teaches or partakes of this (Pravargya), enters that life, and that light. The observance of the rule thereof (is as follows). Let him not cover himself (with a garment) whilst the sun shines, lest he should be concealed from that (sun). Let him not spit whilst the sun shines, lest he should spit upon him. Let him not discharge urine whilst the sun shines, lest he discharge it upon him. For so long as he shines, so great he (the sun) is: thinking, 'Lest I should injure him by these (acts),' let him take food at night, after striking a light, whereby it is made to be a form of him who shines yonder. But on this point Âsuri used to say,--One rule the gods indeed keep, to wit, the truth: let him therefore speak nothing but the truth.