When he heard the sound, he tried to find the speaker, searching on all sides. But when he was unable to find anyone besides himself, he thought it wise to sit down on his lotus seat firmly and give his attention to the execution of penance, as he was instructed.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
निशम्य तद्वक्तृदिदृक्षया दिशो विलोक्य तत्रान्यदपश्यमानः । स्वधिष्ण्यमास्थाय विमृश्य तद्धितं तपस्युपादिष्ट इवादधे मनः
niśamya tadvaktṛdidṛkṣayā diśo vilokya tatrānyadapaśyamānaḥ | svadhiṣṇyamāsthāya vimṛśya taddhitaṃ tapasyupādiṣṭa ivādadhe manaḥ
SB 2.9.7