Srimad Bhagavatam Canto 08, Chapter 24, Text 15

SB 8.24.15

tam atmano ’nugrahartham
 pritya matsya-vapur-dharam
ajanan raksanarthaya
 sapharyah sa mano dadhe
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
To please himself, King Satyavrata, not knowing that the fish was the Supreme Personality of Godhead, decided with great pleasure to give the fish protection.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Here is an example of giving service to the Supreme Personality of Godhead even without knowledge. Such service is called ajñata-sukrti. King Satyavrata wanted to show his own mercy, not knowing that the fish was Lord Visnu. By such unknowing devotional service, one is favored by the Supreme Personality of Godhead. Service rendered to the Supreme Lord, knowingly or unknowingly, never goes in vain.
Srimad Bhagavatam Canto 08, Chapter 24, Text 14
Srimad Bhagavatam Canto 08, Chapter 24, Text 16