SB 8.20.12
yadyapy asav adharmena
mam badhniyad anagasam
tathapy enam na himsisye
bhitam brahma-tanum ripum
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Although He is Visnu Himself, out of fear He has covered Himself in the form of a brahmana to come to me begging. Under the circumstances, because He has assumed the form of a brahmana, even if He irreligiously arrests me or even kills me, I shall not retaliate, although He is my enemy.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
If Lord Visnu as He is had come to Bali Maharaja and asked him to do something, Bali Maharaja certainly would not have refused His request. But to enjoy a little humor between Himself and His devotee, the Lord covered Himself as a brahmana-brahmacari and thus came to Bali Maharaja to beg for only three feet of land.