SB 6.18.58

evaṁ tasyā vrata-sthāyā
 vrata-cchidraṁ harir nṛpa
prepsuḥ paryacaraj jihmo
 mṛga-heva mṛgākṛtiḥ
evam — thus; tasyāḥ — of her; vrata-sthāyāḥ — who was faithfully discharging her vow; vrata-chidram — a fault in the execution of the vow; hariḥ — Indra; nṛpa — O King; prepsuḥ — desiring to find; paryacarat — served; jihmaḥ — deceitful; mṛga- — a hunter; iva — like; mṛga-ākṛtiḥ — in the form of a deer.
O King Parīkṣit, 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.