Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 41

Sanskrit:

रूपमात्राद्विकुर्वाणात्तेजसो दैवचोदितात् ।रसमात्रमभूत्तस्मादम्भो जिह्वा रसग्रह: ॥ ४१ ॥

ITRANS:

rūpa-mātrād vikurvāṇāttejaso daiva-coditātrasa-mātram abhūt tasmādambho jihvā rasa-grahaḥ

Translation:

By the interaction of fire and the visual sensation, the subtle element taste evolves under a superior arrangement. From taste, water is produced, and the tongue, which perceives taste, is also manifested.

Purport:

The tongue is described here as the instrument for acquiring knowledge of taste. Because taste is a product of water, there is always saliva on the tongue.