सदृशं त्रिषु लिङ्गेषु सर्वासु च विभक्तिषु ।
वचनेषु च सर्वेषु यन्न व्येति तदव्ययम् ॥
sadṛśaṁ triṣu liṅgeṣu sarvāsu ca vibhaktiṣu |
vacaneṣu ca sarveṣu yan na vyeti tad avyayam ||
(Prācīna-kārikā; cited in the Amṛta-ṭīkā to Hari-nāmāmṛta-vyākaraṇa: 363, and in Samāsa-cakra)
“That which is the same and does not change in the three genders, in all the cases, and in all numbers is an indeclinable (avyaya).”