As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school, which bases its tenets on the doctrines laid out in the Brahma Sutras, a work by the sage Vyasa that summarizes and systematizes the philosophical teachings of the Upanishads. You can read the Brahma Sutras here. In any case, in Adhyaya 3 Pada 2 Sutra 4 of the Brahma Sutras, Vyasa says this:
sūcakaś ca hi śruter ācakṣate ca tadvidaḥ
(Not altogether) for it (the dream) is indicative (of the future), according to null; the experts also declare this.
This translation is written from the viewpoint of the Advaita philosopher Adi Shankaracharya, who interprets this Sutra as saying that even though dreams are an unreal creation of the Jiva, they still have an element of truth insofar as they can predict the future. (This is in contrast to other commentators who think this Sutra is saying that the fact that dreams predict the future imply that they're created by Paramatma, not the Jiva.) Here is what Adi Shankaracharya says in this section of his Brahma Sutra Bhashya:
Well then, as dreams are mere illusion, they do not contain a particle of reality?--Not so, we reply; for dreams are prophetic of future good and bad fortune. For scripture teaches as follows, 'When a man engaged in some work undertaken for a special wish sees in his dreams a woman, he may infer success from that dream-vision.' Other scriptural passages declare that certain dreams indicate speedy death, so, e.g. 'If he sees a black man with black teeth, that man will kill him.'--Those also who understand the science of dreams hold the opinion that to dream of riding on an elephant and the like is lucky; while it is unlucky to dream of riding on a donkey, &c.; and that certain other dreams also caused by special mantras or devatâs or substances contain a particle of truth.--In all these cases the thing indicated may be real; the indicating dream, however, remains unreal as it is refuted by the waking state.
I discuss the black-skinned man with black teeth in my question and answer here. But now I'm interested in the part in bold in bold. It's understandable that Mantras or gods could cause prophetic dreams. But my question is, what "substances" cause prophetic dreams.
Are these substances ingested, or what? Do any scriptures shed light on this?
