Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 17

Sanskrit:

श्रुत्वा युद्धोद्यमं राम: कुरूणां सह पाण्डवै: ।तीर्थाभिषेकव्याजेन मध्यस्थ: प्रययौ किल ॥ १७ ॥

ITRANS:

śrutvā yuddhodyamaṁ rāmaḥkurūṇāṁ saha pāṇḍavaiḥtīrthābhiṣeka-vyājenamadhya-sthaḥ prayayau kila

Translation:

Lord Balarāma then heard that the Kurus were preparing for war with the Pāṇḍavas. Being neutral, He departed on the pretext of going to bathe in holy places.

Purport:

Both Duryodhana and Yudhiṣṭhira were dear to Lord Balarāma, and so to avoid an awkward situation He departed. Furthermore, after killing the demon Vidūratha, Lord Kṛṣṇa put aside His weapons, but Lord Balarāma still had to kill Romaharṣaṇa and Balvala to finish relieving the earth of her burden of demons.