Srimad-Bhagavatam: Canto 7 - Chapter 1 - Verse 23
Sanskrit:
यदाकल्प: स्वक्रियायां व्याधिभिर्जरयाथवा ।आन्वीक्षिक्यां वा विद्यायां कुर्यादनशनादिकम् ॥ २३ ॥
ITRANS:
yadākalpaḥ sva-kriyāyāṁvyādhibhir jarayāthavāānvīkṣikyāṁ vā vidyāyāṁkuryād anaśanādikam
Translation:
When because of disease or old age one is unable to perform his prescribed duties for advancement in spiritual consciousness or study of the Vedas, he should practice fasting, not taking any food.