Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 21
तप्यमानं त्रिभुवनं प्राणायामैधसाग्निना ।निर्गतेन मुनेर्मूर्ध्न: समीक्ष्य प्रभवस्त्रय: ॥ २१ ॥
tapyamānaṁ tri-bhuvanaṁprāṇāyāmaidhasāgninānirgatena muner mūrdhnaḥsamīkṣya prabhavas trayaḥ
While Atri Muni was engaged in these severe austerities, a blazing fire came out of his head by virtue of his breathing exercise, and that fire was seen by the three principal deities of the three worlds.
According to Śrīla Jīva Gosvāmī, the fire of prāṇāyāma is mental satisfaction. That fire was perceived by the Supersoul, Viṣṇu, and thereby Lord Brahmā and Śiva also perceived it. Atri Muni, by his breathing exercise, concentrated on the Supersoul, or the Lord of the universe. As confirmed in Bhagavad-gītā, the Lord of the universe is Vāsudeva (vāsudevaḥ sarvam iti), and, by the direction of Vāsudeva, Lord Brahmā and Lord Śiva work. Therefore, on the direction of Vāsudeva, both Lord Brahmā and Lord Śiva perceived the severe penance adopted by Atri Muni, and thus they were pleased to come down, as stated in the next verse.