Garudapurana 1.60.18

1.60.18

yasminnṛkṣe vasadbhānustadāndi trīṇi mastake /
trayaṃ vakre pradātavyamekaikaṃ skandhayornyaset //

Starting with the mansion in which the Sun resides, three should be placed on the head;
three should be assigned to the mouth, and one on each of the two shoulders;