Bhagavatapurana 8.19.10

8.19.10

evaṃ sa niścitya ripoḥ śarīram ādhāvato nirviviśe 'surendra /
śvāsānilāntarhitasūkṣmadehas tatprāṇarandhreṇa vivignacetāḥ //

10. O great Ruler of Asuras, having thus resolved, Vishnu entered into the very person of His enemy that rushed towards Him, through his nostrils, with a trembling heart and concealed his subtle body in his breaths (meaning really 'with a unperturbed heart').