CHĀNDOGYA UPANISHAD · 2.22.5

सर्वे स्वरा घोषवन्तो बलवन्तो वक्तव्या इन्द्रे बलं ददानीति ।
सर्व ऊष्माणो ऽग्रस्ता अनिरस्ता विवृता वक्तव्याः प्रजापतेर् आत्मानं परिददानीति ।
सर्वे स्पर्शा लेशेनानभिनिहिता वक्तव्या मृत्योर् आत्मानं परिहराणीति ॥

sarve svarā ghoṣavanto balavanto vaktavyā indre balaṃ dadānīti ।
sarva ūṣmāṇo ’grastā anirastā vivṛtā vaktavyāḥ prajāpater ātmānaṃ paridadānīti ।
sarve sparśā leśenānabhinihitā vaktavyā mṛtyor ātmānaṃ pariharāṇīti ॥

All vowels are to be pronounced with voice (ghosha) and strength (bala), so that the Udgâtri may give strength to Indra. All sibilants are to be pronounced, neither as if swallowed (agrasta), nor as if thrown out (nirasta), but well opened (vivrita), so that the Udgâtri may give himself to Pragâpati. All consonants are to be pronounced slowly, and without crowding them together, so that the Udgâtri may withdraw himself from Mrityu.

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)