Shatapatha Brahmana 8.4.4.11

8.4.4.11

dakṣiṇata u haika upadadhati | tadetāḥ puṇyā lakṣmīrdakṣiṇato dadhmaha iti tasmādyasya dakṣiṇato lakṣma bhavati tam puṇyalakṣmīka ityācakṣata uttarata striyā uttarataāyatanā hi strī tattatkṛtameva purastāttvevainā upadadhyādyatrāhaiva śirastadeva hanū tajjihvāthaitāḥ puṇyā lakṣmīrmukhato dhatte tasmādyasya mukhe lakṣma bhavati tam puṇyalakṣmīka ityācakṣate

Some, again, lay them down on the right (south) side, saying, 'We thus place these signs of good fortune (punyâ lakshmî) on the right side:' whence he who has a mark (lakshman) on his right side is said to have good luck (punya-lakshmîka), and on the left side in the case of a woman; for the woman has her position on the left side (of the man): therefore it is done thus. But let him place them in front; for where the head is there are also the jaws and the tongue: and thus he places the signs of good fortune at the head (or, in the mouth, mukhatah), whence they say that he who has a (peculiar) mark in his mouth has good luck.