من رآني فقد رأى الحق

"Whoever sees me (in his dream), then he has in fact seen me." (Al-Bukhari no. 6996 and Muslim no. 2267)

— is authentic, and there are various wordings for it, in one narration:

من رآني في المنام فقد رآني فإن الشيطان لا يتمثل في صورتي

"Whoever sees me in his sleep, then he has indeed seen me, for the Shaytan cannot impersonate me in my appearance." (Al-Bukhari no. 110)

And in another:

من رآني في المنام فقد رأى الحق ان الشيطان لا يتشبه بي

"Whoever sees me in his sleep, then he has in fact seen me, for the Shaytan cannot impersonate me in my appearance." (Ahmad 2/261)

There are many different wordings for this Hadeeth, and each one confirms that the Shaytan, the enemy of Allaah, cannot take on the Prophet's appearance, and so when a person has a dream about the Prophet (sallAllaahu `alayhi wa sallam), he has truly seen him in the dream.

The Prophet's appearance is well-known among scholars: He was of medium height, handsome, with a reddish complexion and a thick black beard; however, later on in his life, a few of his hairs turned white.

As for the second Hadeeth -

من رآني فقد حرمت عليه النار

"Whoever sees me (in his dream), then the Fire is forbidden for him."

— it has no basis and is not authentic.