SB 6.18.58
evam tasya vrata-sthaya
vrata-cchidram harir nrpa
prepsuh paryacaraj jihmo
mrga-heva mrgakrtih
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O King Pariksit, as the hunter of a deer becomes like a deer by covering his body with deerskin and serving the deer, so Indra, although at heart the enemy of the sons of Diti, became outwardly friendly and served Diti in a faithful way. Indra’s purpose was to cheat Diti as soon as he could find some fault in the way she discharged the vows of the ritualistic ceremony. However, he wanted to be undetected, and therefore he served her very carefully.