SB 10.78.38

ṛṣaya ūcuḥ
ilvalasya suto ghoro
 balvalo nāma dānavaḥ
sa dūṣayati naḥ satram
 etya parvaṇi parvaṇi
Word for word: 
ṛṣayaḥ ūcuḥ — the sages said; ilvalasya — of Ilvala; sutaḥ — the son; ghoraḥ — fearsome; balvalaḥ nāma — named Balvala; dānavaḥ — demon; saḥ — he; dūṣayati — contaminates; naḥ — our; satram — sacrifice; etya — coming; parvaṇi parvaṇi — on each new-moon day.
Translation: 
The sages said: A fearsome demon named Balvala, the son of Ilvala, comes here every new-moon day and contaminates our sacrifice.
Purport: 

First the sages tell Lord Balarāma the favor they would like Him to do for them.