BṚHADĀRAṆYAKA UPANISHAD · 4.5.9

स यथा शङ्खस्य ध्मायमानस्य न बाह्याञ् छब्दाञ् छक्नुयाद् ग्रहणाय ।
शङ्खस्य तु ग्रहणेन शङ्खध्मस्य वा शब्दो गृहीतः ॥

sa yathā śaṅkhasya dhmāyamānasya na bāhyāñ chabdāñ chaknuyād grahaṇāya ।
śaṅkhasya tu grahaṇena śaṅkhadhmasya vā śabdo gṛhītaḥ ॥

'And as the sounds of a conch-shell, when blown, cannot be seized externally (by themselves), but the sound is seized, when the shell is seized, or the blower of the shell;