The virtue of ‘Ali ibn Abi Taalib and the ruling on sending blessings exclusively upon him
Question

                What is the ruling on saying “sall-Allaahu ‘alayhi wa sallam (may Allaah send blessings and peace upon him)” with regard to ‘Ali ibn Abi Taalib? Is that correct or not?
Praise be to Allah. 
          Shaykh al-Islam Ibn Taymiyah was asked about a man who 
          said that ‘Ali ibn Abi Taalib was not one of the Ahl al-Bayt (members 
          of the Prophet’s household) and that it is not permissible to send blessings 
          upon him, and that sending blessings upon him was bid’ah (an innovation). 
 
          He replied: 
 
          With regard to ‘Ali ibn Abi Taalib being one of the Ahl 
          al-Bayt, this is something concerning which there is no difference of 
          opinion among the Muslims. It is so obvious to the Muslims that there 
          is no need for evidence to prove it. Rather he is the best of the Ahl 
          al-Bayt, the best of Bani Haashim after the Prophet 
          
          (peace and blessings of Allaah be upon him). It was proven that the 
          Prophet 
          
          (peace and blessings of Allaah be upon him) threw his cloak over ‘Ali, 
          Faatimah, Hasan and Husayn and said, “O Allaah, these are the members 
          of my household, so to remove  Ar-Rijs (evil deeds and 
          sins) from them and purify them with a thorough purification [cf. al-Ahzaab 
          33:33].” 
 
          With regard to sending blessings exclusively upon him, 
          this issue leads us to another issue, which is whether it is permissible 
          to send blessings exclusively upon anyone other than the Prophet 
          
          (peace and blessings of Allaah be upon him), such as saying, “O Allaah 
          send blessings on ‘Umar, or ‘Ali (Allaahumma salli ‘ala ‘Umar 
          etc.)”. The scholars disputed concerning that. Maalik, al-Shaafa’i and 
          a group of the Hanbalis said that we should not send blessings exclusively 
          upon anyone except the Prophet 
          
          (peace and blessings of Allaah be upon him), as it was narrated that 
          Ibn ‘Abbaas said: “I do not know that blessings should be sent upon 
          anyone except the Prophet 
          
          (peace and blessings of Allaah be upon him). Imaam Ahmad and most of 
          his companions said that there was nothing wrong with that, because 
          ‘Ali ibn Abi Taalib said to ‘Umar ibn al-Khattaab, “May Allaah send 
          blessings upon you.” This is the more correct view. But singling out 
          one of the Sahaabah or relatives of the Prophet 
          
          (peace and blessings of Allaah be upon him), such as ‘Ali, or anyone 
          else, to send blessings upon them, is a form of making them rival the 
          Prophet 
          
          (peace and blessings of Allaah be upon him), in such a way that this 
          phrase becomes like a symbol or slogan that is attached to his name, 
          this is what is bid’ah.