What is the Buddhist Perspective on Dreaming? I met my family who has already died in my dream. He told me something bad about my partner.
Should I believe him?
According to the Milindapañha there are six causes of dreams:


vātiko supinaṃ passati - one with winds sees a dream
pittiko supinaṃ - one with bile sees a dream
semhiko supinaṃ passati - one with phlegm sees a dream
devatūpasaṃhārato supinaṃ passati - one possessed by a devata sees a dream
samudāciṇṇato supinaṃ passati - I don't know what this one means, maybe something to do with memories?
pubbanimittato supinaṃ passati - one sees a dream as a prophetic vision



  tatra, mahārāja, yaṃ pubbanimittato supinaṃ passati, taṃ yeva saccaṃ, avasesaṃ micchā”ti.


Of these six, only the last type is true, the rest are false. 

I think it is possible that possession by a devata could lead to dreams containing truth, but it is just as likely to be misleading. How would you know if your were being deceived? Much better to rely on waking experience. Nagasena says mostly dreams occur when one is kapiniddāpareta - afflicted with monkey sleep.

Source: milindapañha, anumānapañho, vessantaravaggo, supinapañho