SB 4.19.21

atriṇā coditas tasmai
 sandadhe viśikhaṁ ruṣā
so ’śvaṁ rūpaṁ ca tad dhitvā
 tasthāv antarhitaḥ svarāṭ
Word for word: 
atriṇā — by the great sage Atri; coditaḥ — inspired; tasmai — for Lord Indra; sandadhe — fixed; viśikham — his arrow; ruṣā — out of great anger; saḥ — King Indra; aśvam — horse; rūpam — the dress of a sannyāsī; ca — also; tat — that; hitvā — giving up; tasthau — he remained there; antarhitaḥ — invisible; sva-rāṭ — the independent Indra.
Translation: 
When the great sage Atri again gave directions, the son of King Pṛthu became very angry and placed an arrow on his bow. Upon seeing this, King Indra immediately abandoned the false dress of a sannyāsī and, giving up the horse, made himself invisible.