Narapatijayacarya 3.1.3.57

3.1.3.57

pūrvasminanasvaraḥ svāmī ikāro dakṣiṇe tathā ||
ukāro paścime jñeyaṃ e saumṛye madhya osvaraḥ ||

The vowel 'a' is the lord in the east; likewise, the vowel 'i' is in the south;
the vowel 'u' is to be known in the west; 'e' is in the north; the sound 'o' is in the center.