All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
  is His slave and Messenger.
'Aa'ishah 
  narrated that the Prophet 
  used to say Tasleem once while facing straight and leaning slightly to the right side. [Al-Haakim and At-Tirmithi] 
Al-Haakim 
  classified this narration to be authentic according to the conditions set by Al-Bukhari and Muslim 
  and Ath-Thahabi 
  is of the same view. In addition to this, Al-Albaani 
  classified it as authentic. This narration proves that it is permissible to conclude the prayer with one Tasleem. 
This narration is the same as that narrated by Anas 
  that the Prophet 
  used to conclude the prayer with one Tasleem. [At-Tabaraani, and Al-Albaani classified it as authentic.]  
The majority of the scholars 
  are of the view that it is acceptable to conclude the prayer with one Tasleem based on these two narrations.
However, in general, the Prophet 
  used to conclude the prayer saying Tasleem twice, one to his right hand side and one to his left hand side. Sa'd 
  said: "I used to see the Prophet 
  saying Tasleem to his right hand side and to his left hand side until I could see the whiteness of his cheek [while performing the prayer behind him]." [Ahmad, Muslim, An-Nasaa'i and others]