Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 48
Sanskrit:
प्रतिपद्दिनमारभ्य यावच्छुक्लत्रयोदशीम् ।ब्रह्मचर्यमध:स्वप्नं स्नानं त्रिषवणं चरेत् ॥ ४८ ॥
ITRANS:
pratipad-dinam ārabhyayāvac chukla-trayodaśīmbrahmacaryam adhaḥ-svapnaṁsnānaṁ tri-ṣavaṇaṁ caret
Translation:
From pratipat until the thirteenth day of the next bright moon [śukla-trayodaśī], one should observe complete celibacy, sleep on the floor, bathe three times a day and thus execute the vow.