SB 4.19.21
atrina coditas tasmai
sandadhe visikham rusa
so ’svam rupam ca tad dhitva
tasthav antarhitah svarat
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the great sage Atri again gave directions, the son of King Prthu became very angry and placed an arrow on his bow. Upon seeing this, King Indra immediately abandoned the false dress of a sannyasi and, giving up the horse, made himself invisible.