SB 10.20.18

dhanur viyati māhendraṁ
 nirguṇaṁ ca guṇiny abhāt
vyakte guṇa-vyatikare
 ’guṇavān puruṣo yathā
Synonyms: 
dhanuḥ — the bow (rainbow); viyati — within the sky; māhā-indram — of Lord Indra; nirguṇam — without qualities (or without a bowstring); ca — although; guṇini — within the sky, which has definite qualities like sound; abhāt — appeared; vyakte — within the manifest material nature; guṇa-vyatikare — which consists of the interactions of material qualities; aguṇa-vān — He who has no contact with material qualities; puruṣaḥ — the Supreme Personality; yathā — just as.
Translation: 
When the curved bow of Indra [the rainbow] appeared in the sky, which had the quality of thundering sound, it was unlike ordinary bows because it did not rest upon a string. Similarly, when the Supreme Lord appears in this world, which is the interaction of the material qualities, He is unlike ordinary persons because He remains free from all material qualities and independent of all material conditions.
Purport: 

Śrīla Prabhupāda comments as follows: “Sometimes, in addition to the roaring thunder of the clouds, there is an appearance of a rainbow, which stands as a bow without a string. Actually, a bow is in the curved position because it is tied at its two ends by the bowstring; but in the rainbow there is no such string, and yet it rests in the sky so beautifully. Similarly, when the Supreme Personality of Godhead descends to this material world, He appears just like an ordinary human being, but He is not resting on any material condition. In the Bhagavad-gītā, the Lord says that He appears by His internal potency, which is free from the bondage of the external potency. What is bondage for the ordinary creature is freedom for the Personality of Godhead.”