SB 6.12.18
sri-suka uvaca
indro vrtra-vacah srutva
gatalikam apujayat
grhita-vajrah prahasams
tam aha gata-vismayah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sukadeva Gosvami said: Hearing the straightforward, instructive words of Vrtrasura, King Indra praised him and again took the thunderbolt in his hand. Without bewilderment or duplicity, he then smiled and spoke to Vrtrasura as follows.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
King Indra, the greatest of the demigods, was astonished to hear the instructions of Vrtrasura, who was supposed to be a demon. He was struck with wonder that a demon could speak so intelligently. Then he remembered great devotees like Prahlada Maharaja and Bali Maharaja, who had been born in the families of demons, and thus he came to his senses. Even so-called demons sometimes have exalted devotion for the Supreme Personality of Godhead. Therefore Indra smiled reassuringly at Vrtrasura.