Charak Samhita
इह खलु ज्वर एवादौ विकाराणामुपदिश्यते, तत्प्रथमत्वाच्छारीराणाम् ॥१६॥
Jwara (fever) is described first among disorders because temperature (or body heat) is a life-sustaining force, and is the first condition (afflicting patients of somatic conditions).
english translation
iha khalu jvara evAdau vikArANAmupadizyate, tatprathamatvAcchArIrANAm ॥16॥
hk transliteration by Sanscript