Shiva Purana

siva.sh/siva-purana/uma-samhita/36/58

श्रुत्वा तद्वृत्तमाज्ञाय तं शशाप कृतागसम् ।। अकामतोविचार्य्येति शूद्रो भव न क्षत्रियः ।। ५८।।

sanskrit

On hearing about the incident and realising that he had committed a sin unconsciously and not wilfully, the preceptor cursed him—“Become a Śūdra, not a Kṣatriya.”

english translation

zrutvA tadvRttamAjJAya taM zazApa kRtAgasam || akAmatovicAryyeti zUdro bhava na kSatriyaH || 58||

hk transliteration by Sanscript