Srimad-Bhagavatam: Canto 7 - Chapter 1 - Verse 5
सायं प्रातश्चरेद्भैक्ष्यं गुरवे तन्निवेदयेत् ।भुञ्जीत यद्यनुज्ञातो नो चेदुपवसेत् क्वचित् ॥ ५ ॥
sāyaṁ prātaś cared bhaikṣyaṁgurave tan nivedayetbhuñjīta yady anujñātono ced upavaset kvacit
The brahmacārī should go out morning and evening to collect alms, and he should offer all that he collects to the spiritual master. He should eat only if ordered to take food by the spiritual master; otherwise, if the spiritual master does not give this order, he may sometimes have to fast.