Shama-il Tirmidhi Chapter 055, Hadith Number 002 (390).

Abu Hurayrah Radiyallahu 'Anhu reports that Rasulullah Sallallahu 'Alayhi Wasallam said: "The one who sees me in one's dream, has actually seen me, because the shaytaan cannot imitate my person".

Commentary
The manner in which Allah Ta'aala has protected Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam from the shaytaan during his life time, in the same manner, after his death the shaytaan has not been given the power to impersonate Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam. This is a decided fact. Now the question may arise that is the original form of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam seen?, that is, does the one who sees him obtain such strength that one will see him in his actual form, or does one see only his image. For example, if a person is sitting a little away from a mirror, another person who is also at a distance can see the image of the person in the mirror, but due to some obstruction cannot see the actual person. The sufis opine that Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam is seen both ways. Some people actually see Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam and some see his image, like the one described of the mirror. For this reason at times Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam is seen in the form of others, as if that person is a mirror of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam.