Reference: http://binothaimeen.net/content/6668
Fataawa noor ‘alad-Darb – Tape No.4
Question: A man saw our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) in his dream, and he said to him: ‘Pray for me O Messenger of Allaah’. So is this dream real, or [just] imaginary? We request your guidance, and may Allaah grant you success.
Response: Seeing the Prophet (sal-Allaahu ‘alayhi wa sallam) [in a dream] is true, because the Shaytaan is unable to appear in his form. However, it is necessary to compare the description of who was seen [in the dream] to that which is mentioned in the ahaadeeth about the description of the Messenger of Allaah (sal-Allaahu ‘alayhi wa sallam). So, if the description [of who was seen in the dream] matches the description of the Prophet sal-Allaahu ‘alayhi wa sallam, then it is true, and if it is contrary to what he saw, then [who] he [saw] was not the Prophet (sal-Allaahu ‘alayhi wa sallam).
And some of the people experience this a lot; they see something from their imagination, and believe it to be the Prophet (sal-Allaahu ‘alayhi wa sallam), or it is said to them that he is the Prophet (sal-Allaahu ‘alayhi wa sallam). So when they describe who they saw, the description is contrary to the description of the Prophet (sal-Allaahu ‘alayhi wa sallam) – and with that, it is clear what he saw was not true[ly the Prophet (sal-Allaahu ‘alayhi wa sallam)].
Whatever the case, it is imperative that he ascertain the description of who he saw in the dream matches the description of the Prophet (sal-Allaahu ‘alayhi wa sallam); and if it does not match, then who he saw [in his dream] was not the Prophet (sal-Allaahu ‘alayhi wa sallam).