Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 51
Sanskrit:
नाधौतपादाप्रयता नार्द्रपादा उदक्शिरा: ।शयीत नापराङ्नान्यैर्न नग्ना न च सन्ध्ययो: ॥ ५१ ॥
ITRANS:
nādhauta-pādāprayatānārdra-pādā udak-śirāḥśayīta nāparāṅ nānyairna nagnā na ca sandhyayoḥ
Translation:
You should not lie down without having washed both of your feet or without being purified, nor with wet feet or with your head pointed west or north. You should not lie naked, or with other women, or during the sunrise or sunset.