Śrīmad-Bhāgavatam · Text 9.16.33
Who is speaking and story frame
ये मधुच्छन्दसो ज्येष्ठा: कुशलं मेनिरे न तत् ।
अशपत् तान्मुनि: क्रुद्धो म्लेच्छा भवत दुर्जना: ॥ ३३ ॥
ye madhucchandaso jyeṣṭhāḥ
kuśalaṁ menire na tat
aśapat tān muniḥ kruddho
mlecchā bhavata durjanāḥ
When requested by their father to accept Śunaḥśepha as the eldest son, the elder fifty of the Madhucchandās, the sons of Viśvāmitra, did not agree. Therefore Viśvāmitra, being angry, cursed them. “May all of you bad sons become mlecchas,” he said, “being opposed to the principles of Vedic culture.”
Read Śrīla Prabhupāda's purport on Vedabase.io