SB 10.16.22

kṛṣṇa-prāṇān nirviśato
 nandādīn vīkṣya taṁ hradam
pratyaṣedhat sa bhagavān
 rāmaḥ kṛṣṇānubhāva-vit
kṛṣṇa-prāṇān — the men whose very life and soul was Kṛṣṇa; nirviśataḥ — entering; nanda-ādīn — headed by Nanda Mahārāja; vīkṣya — seeing; tam — that; hradam — lake; pratyaṣedhat — forbade; saḥ — He; bhagavān — the all-powerful Lord; rāmaḥ — Balarāma; kṛṣṇa — of Lord Kṛṣṇa; anubhāva — the power; vit — knowing well.
Lord Balarāma then saw that Nanda Mahārāja and the other cowherd men, who had dedicated their very lives to Kṛṣṇa, were beginning to enter the serpent’s lake. As the Supreme Personality of Godhead, Lord Balarāma fully knew Lord Kṛṣṇa’s actual power, and therefore He restrained them.

Śrīla Sanātana Gosvāmī explains that Lord Balarāma checked some of the cowherd men by speaking to them, others by physically holding them and still others by casting upon them His potent smiling glance. Distraught over the situation, they were prepared to give up their lives for Lord Kṛṣṇa by entering the serpent’s lake.