CC Madhya 20.355

yasyāvatārā jñāyante
śarīreṣv aśarīriṇaḥ
tais tair atulyātiśayair
vīryair dehiṣv asaṅgataiḥ
Word for word: 
yasya — whose; avatārāḥ — incarnations; jñāyante — can be known; śarīriṣu — among the living entities; aśarīriṇaḥ — of the Lord, who has no material body; taiḥ taiḥ — all those; atulya — incomparable; atiśayaiḥ — extraordinary; vīryaiḥ — by prowess; dehiṣu — among the living entities; asaṅgataiḥ — impossible.
Translation: 
“ ‘The Lord does not have a material body, yet He descends among human beings in His transcendental body as an incarnation. Therefore it is very difficult for us to understand who is an incarnation. Only by His extraordinary prowess and uncommon activities, which are impossible for embodied living entities, can one partially understand the incarnation of the Supreme Personality of Godhead.’
Purport: 

This is a quotation from Śrīmad-Bhāgavatam (10.10.34).