BṚHADĀRAṆYAKA UPANISHAD · 1.3.19

सो ऽयास्य आङ्गिरसो ऽङ्गानां हि रसः ।
प्राणो वा अङ्गानां रसः ।
प्राणो हि वा अङ्गानां रसः ।
तस्माद् यस्मात् कस्माच्चाङ्गात् प्राण उत्क्रामति तद् एव तच् छुष्यति ।
एष हि वा अङ्गानां रसः ॥

so ’yāsya āṅgiraso ’ṅgānāṃ hi rasaḥ ।
prāṇo vā aṅgānāṃ rasaḥ ।
prāṇo hi vā aṅgānāṃ rasaḥ ।
tasmād yasmāt kasmāccāṅgāt prāṇa utkrāmati tad eva tac chuṣyati ।
eṣa hi vā aṅgānāṃ rasaḥ ॥

He was called Ayâsya Âṅgirasa, for he is the sap (rasa) of the limbs (aṅga). Verily, breath is the sap of the limbs. Yes, breath is the sap of the limbs. Therefore from whatever limb breath goes away, that limb withers, for breath verily is the sap of the limbs.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)