Māṇḍūkī Śikṣā 87

87

halantāduttaro yastu padādavagraheṣu ca
yasya svasmādya ityeṣo yonyaḥ sa ya iti smṛtaḥ

That which follows a consonant-ending, and occurs at word-division in avagrahas, / which comes from its own source as y, is considered as original y.