Brihat Parashara Hora Shastra
षष्ठाष्टव्ययनाथेषु यो बली विषमर्क्षगः । पृष्ठस्थितो भवेद् ब्रह्मां बलिनो लग्नजाययोः ॥ १७१॥
From amongst the Lords of Ari, Randhr and Vyaya the Grah, who may possess the greatest strength and is placed in Lagn, or in Yuvati with strength, the one, placed in an odd Rāśi within sixth Bhava from the Bhava concerned, is called the Brahm Grah.
english translation
SaSThASTavyayanAtheSu yo balI viSamarkSagaH । pRSThasthito bhaved brahmAM balino lagnajAyayoH ॥ 171॥
hk transliteration by Sanscript