pāṇinā prāha vismitaḥ

naradah-the great sage Narada; tat-that; upakarnya-overhearing; jnatva-and knowing; tasya-his (Dhruva Maharaja's); cikirsitam-activities; sprstva-by touching; murdhani-on the head; agha-ghnena-which can drive away all sinful activities; panina-by the hand; praha-said; vismitah-being surprised.