Manava Dharmashastra 4.104

4.104

etāṃs tv abhyuditān vidyād yadā prāduṣkṛtāgniṣu
tadā vidyād anadhyāyam anṛtau cābhradarśane //

When one perceives these (phenomena) all together (in the twilight), after the sacred fires have been made to blaze (for the performance of the Agnihotra), then one must know the recitation of the Veda to be forbidden, and also when clouds appear out of season.