BṚHADĀRAṆYAKA UPANISHAD · 2.4.7

स यथा दुन्दुभेर् हन्यमानस्य न बाह्याञ् छब्दाञ् छक्नुयाद् ग्रहणाय ।
दुन्दुभेस् तु ग्रहणेन दुन्दुभ्याघातस्य वा शब्दो गृहीतः ॥

sa yathā dundubher hanyamānasya na bāhyāñ chabdāñ chaknuyād grahaṇāya ।
dundubhes tu grahaṇena dundubhyāghātasya vā śabdo gṛhītaḥ ॥

'Now as the sounds of a drum, when beaten, cannot be seized externally (by themselves), but the sound is seized, when the drum is seized or the beater of the drum;