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.