The Compendium of Caraka 1.8.24

1.8.24

na vidyutsvanārtavīṣu nābhyuditāsu dikṣu nāgnisaṃplave na bhūmikampe na mahotsave nolkāpāte na mahāgrahopagamane na naṣṭacandrāyāṃ tithau na sandhyayornāmukhādgurornāvapatitaṃ nātimātraṃ na tāntaṃ na visvaraṃ nānavasthitapadaṃ nātidrutaṃ na vilambitaṃ nātiklībaṃ nātyuccarinātinīcaiḥ svarairadhyayanamabhyasyet //

One should not practice study during unseasonal lightning, or when the directions glow bright red, or during a fire outbreak, or during an earthquake, or during a great festival, or during a meteor fall, or during an occultation by major planets, or on a moonless day, or at the twilights, nor without receiving it from the mouth of a teacher, nor in an unaccented voice, nor excessively, nor when exhausted, nor out of tune, nor with irregular pauses, nor too quickly, nor too slowly, nor too weakly, nor in tones that are too high or too low.