Jambuddivapannatti 2.28

2.28

sattheṇa sutikkheṇavi, chettuṃ bhittuṃ ca jaṃ kira ṇa sakkā |
taṃ paramāṇuṃ siddhā, vayaṃti ādiṃ pamāṇāṇaṃ ||

What indeed cannot be cut or split even by the sharpest weapon,
that atom the Siddhas declare to be the beginning of measurements.