Yāska's Nirukta 42.22

42.22

"athāta upamāḥ"| atha idānīm, ataḥ anantaram, upamāḥ bhavanti | tadyathā, "idamiva" 'idaṃ yathā,' 'agnina̍ye,' 'caturaścid dadamānāt,' 'brāhmaṇā vratacāriṇaḥ,' 'vṛkṣasya nu te puruhūta vayāḥ', 'jāra ā bhagam', 'meṣo bhūto'bhi yannayaḥ,' 'tadrūpaḥ,' tadvarṇaḥ, tadvat, tathā"-etā upamāḥ || sāmānyalakṣaṇamāsāṃ bravīti,- "yadatattatsadṛśamiti gārgyaḥ" | yat kiñcit arthajātam atat bhavati, tat sadṛśamiti sarūpaṃ ca, yathā-anāgniḥ khadyotaḥ, agnisarūpaśca so'gninopamīyate -'agniriva khadyotaḥ' iti evam atatsarūpeṇa guṇena guṇasāmānyāt upamīyate, ityevaṃ gārgyaḥ, ācāryoṃ manyate |