Canto 3 -
Araṇya-kāṇḍa
Chapter 11: Rāma Goes to Agastya’s Āśrama
Text 3.11.58

धारयन्ब्राह्मणं रूपमिल्वलः संस्कृतं वदन्।
आमन्त्रयति विप्रान्स्म श्राद्धमुद्दिश्य निर्घृणः॥

dhārayan brāhmaṇaṁ rūpam ilvalaḥ saṁskṛtaṁ vadan
āmantrayati viprān sma śrāddham uddiśya nirghṛṇaḥ

dhārayan = assuming; brāhmaṇam = of a brāhmaṇa; rūpam = the form; ilvalaḥ = Ilvala; saṁskṛtam = in Sanskrit; vadan = and speaking; āmantrayati viprān sma = would invite vipras; śrāddham = śrāddha; uddiśya = for; nirghṛṇaḥ = the merciless.

Assuming the form of a brāhmaṇa and speaking in Sanskrit, the merciless Ilvala would invite vipras for śrāddha.

Saṁskṛtaṁ vadan indicates that [like brāhmaṇas] Ilvala would speak in that language whose grammar was refined.1

1 Rāmāyaṇa-bhūṣaṇa: saṁskṛtaṁ vyākaraṇa-saṁskāravatīṁ vācaṁ vadan. Technical note: saṁskṛtaṁ brāhmaṇavad eva saṁskṛta-bhāṣāṁ vadan.