Sayyiduna Anas (رضي الله عنه) mentions that he went with a man to the market, when they noticed a particular item being sold.
[Sayyiduna Anas says] I enquired about it, the seller asked for thirty. The man [that I was with] looked at it and said: ‘I will take it for forty.’ The seller asked: ‘What makes you do this, whereas I am giving it to you for less?’ The man took another look and said: ‘I will take it for fifty.’
The seller asked: ‘What makes you do this, whereas I’m giving it to you for less?’ He replied: *‘I heard Rasulullah (صلى الله عليه وسلم) saying: ‘A bondsman will not be a true believer until he loves for his brother what he loves for himself’, and I see this to be worth fifty.*