Shiva Purana

siva.sh/siva-purana/saturda-samhita/19/62

मुनी रथात्समुत्तीर्य दृष्ट्वा तां दृढताम्पराम् ।। तस्मै भूत्वा सुप्रसन्नो वज्राङ्गत्ववरन्ददौ ।। ६२।।

sanskrit

The sage descended the chariot. He was delighted at the steadfastness of Kṛṣṇa and blessed him with the boon that his body would become adamantine.

english translation

munI rathAtsamuttIrya dRSTvA tAM dRDhatAmparAm || tasmai bhUtvA suprasanno vajrAGgatvavarandadau || 62||

hk transliteration by Sanscript