SB 4.1.16

vidura uvāca
atrer gṛhe sura-śreṣṭhāḥ
 sthity-utpatty-anta-hetavaḥ
kiñcic cikīrṣavo jātā
 etad ākhyāhi me guro
Word for word: 
viduraḥ uvāca — Śrī Vidura said; atreḥ gṛhe — in the house of Atri; sura-śreṣṭhāḥ — chief demigods; sthiti — maintenance; utpatti — creation; anta — destruction; hetavaḥ — causes; kiñcit — something; cikīrṣavaḥ — desiring to do; jātāḥ — appeared; etat — this; ākhyāhi — tell; me — to me; guro — my dear spiritual master.
Translation: 
After hearing this, Vidura inquired from Maitreya: My dear master, how is it that the three deities Brahmā, Viṣṇu and Śiva, who are the creator, maintainer and destroyer of the whole creation, became the offspring of the wife of Atri Muni?
Purport: 

The inquisitiveness of Vidura was quite fitting, for he understood that when the Supersoul, Lord Brahmā and Lord Śiva all appeared through the person of Anasūyā, the wife of Atri Muni, there must have been some great purpose. Otherwise why should they have appeared in such a way?