Upon hearing the curse of the sages, the terrified boys quickly uncovered the belly of Sāmba, and indeed they observed that therein was an iron club.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तच्छ्रुत्वा तेऽतिसन्त्रस्ता विमुच्य सहसोदरम् । साम्बस्य ददृशुस्तस्मिन् मुसलं खल्वयस्मयम्
tacchrutvā te'tisantrastā vimucya sahasodaram | sāmbasya dadṛśustasmin musalaṃ khalvayasmayam
SB 11.1.17