Mahabharata — Anushasana Parva, Shloka 140
ज्येष्ठ-कनिष्ठ-धर्मः — Duties of Elders and Juniors
Anuśāsana-parva 108
संध्यायां च न भुज्जीत न स्नायेन्न तथा पठेत् । प्रयतश्न॒ भवेत् तस्यां न च किंचित् समाचरेत्
saṃdhyāyāṃ ca na bhuñjīta na snāyen na tathā paṭhet | prayataś ca bhavet tasyāṃ na ca kiṃcit samācaret ||
毗湿摩说道：在暮昏之际（saṃdhyā），不应进食，不应沐浴，亦不应诵读学习。于此神圣的间隙，当自持其身，澄净其心，专注观想与礼拜，不可另作他事。
भीष्म उवाच
Educational Q&A
Twilight (saṃdhyā) is to be treated as a dedicated sacred interval: avoid ordinary activities like eating, bathing, and study, and instead remain disciplined and focused on worship/meditation, doing nothing else.
In the Anuśāsana Parva’s dharma-instruction setting, Bhīṣma continues prescribing rules of right conduct, here specifying proper behavior during the saṃdhyā period as part of daily religious discipline.