Srimad Bhagavatam Canto 10, Chapter 88, Text 22

SB 10.88.22

tac chrutva bhagavan rudro
 durmana iva bharata
om iti prahasams tasmai
 dade ’her amrtam yatha
 
Translation: 
 
Upon hearing this, Lord Rudra seemed somewhat disturbed. Nonetheless, O descendant of Bharata, he vibrated om to signify his assent, granting Vrka the benediction with an ironic smile, as if giving milk to a poisonous snake.
Srimad Bhagavatam Canto 10, Chapter 88, Text 21
Srimad Bhagavatam Canto 10, Chapter 88, Text 23