Source: The Great Interpretation of Dreams (Ibn Sirin)
The speech of a dead person in a dream is truth and sincerity, because they are in the abode of truth. Everything they say, command, or forbid is true and must be followed if it is good. Whoever sees himself speaking to a dead person will have a long life.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
Whoever sees a dead person speaking to him will have reconciliation with people after a dispute, or a difficult need will be met. The speech of the dead is an admonition and a reminder.
Source: Signs in the Science of Expressions (Ibn Shaheen)
The speech of the dead indicates an admonition for the dreamer. If they command something, it must be done if it is good and in accordance with Sharia. If they forbid something, it must be avoided.