Śaunaka Śikṣā 23

23

nakārasya padāntasya rephontasthāsvaroṣmasu
lopastvākārapūrvasya saśau tu tacayoḥ pare

A final 'na' followed by 'ra', a semivowel, a vowel, or a spirant / is elided when preceded by 'ā'; but before 'ta' and 'ca', [it becomes] 'sa' and 'śa' [respectively].