Musnad Ahmad 1091:2

Aswad ibn Amir narrated to us, Abu Bakr informed us, from al-A’mash, from Abu Khalid al-Walibi, from al-Nu’man ibn Muqarrin al-Muzani, who said: The Messenger of Allah (ﷺ) said: A man insulted another man in his presence. The man who was being insulted began saying: "Upon you be peace" (Alayka al-salam). The Messenger of Allah (ﷺ) said: "As for that, an angel is between you two, defending you. Every time this one insults you, he says to him: 'Rather, it is you, and you are more deserving of it.' And when he says to him: 'Upon you be peace,' he says: 'No, rather it is for you; you are more deserving of it.'"