I found the following point of benefit in the work of the great Hanbali Sunni, Imam al-Saffarini al-Hanbali (d.1188) from his work entitled ‘Ghidha al-Albab’ (1/24-25 ). I had been wanting to post something on this for some time, not least due to many of us having inherited this practice due to hearing it so often from our elders, only to have some well meaning people label it as incorrect. It is hoped the passage below will foster greater tolerance and understanding of difference of opinion amongst Muslims. Imam al-Saffarini said:
Ibn Kathir mentioned that it is common in the writing of many of the scribes of books to single out Ali (Allah be pleased with him) that they say ‘Upon him be peace’ عليه السلام for him and not the other companions, or ‘Allah ennoble his face’ كرم الله وجهه. Although the meaning of this is correct, the companions should be treated equally in this, for this is from the point of respect and honouring, and the two Shaykhs and the commander of the believers Uthman are more deserving of this than him. [end of quote]
I say [Imam al-Saffarini]: this is common and widespread and has filled the books and the ears (of people). The Shaykhs have said: Ali (Allah be pleased with him) is specified with the statement ‘Allah ennoble his face’ كرم الله وجهه because he never prostrated to an idol, and there is no harm in this -Allah Most High willing-. And Allah is the giver of success.
Note: Also refer to Shaykh Faraz Rabbani’s answer here