SB 8.24.29

sarve līlāvatārās te
 bhūtānāṁ bhūti-hetavaḥ
jñātum icchāmy ado rūpaṁ
 yad-arthaṁ bhavatā dhṛtam
Word for word: 
sarve — everything; līlā — pastimes; avatārāḥ — incarnations; te — of Your Lordship; bhūtānām — of all living entities; bhūti — of a flourishing condition; hetavaḥ — the causes; jñātum — to know; icchāmi — I wish; adaḥ — this; rūpam — form; yat-artham — for what purpose; bhavatā — by Your Lordship; dhṛtam — assumed.
Translation: 
All Your pastimes and incarnations certainly appear for the welfare of all living entities. Therefore, my Lord, I wish to know the purpose for which You have assumed this form of a fish.