Al-Matalib al-'Aliyah 4353 — The command to refrain from fighting

It is narrated from the paternal uncle of Amr that he said: I went out with Muslim bin Uqbah. When we came level with a valley where Sayyiduna Muhammad bin Maslamah (may Allah be pleased with him) was staying, Muslim sent me to him. I asked (before going): If he refuses to come to you, what is your order? He said: "Then cut off his head and bring it to me." So I went to him and said: Respond to the summons of the amir (come with me to him). He asked: "Who is the amir?" I said: Muslim bin Uqbah. He said: "What does the amir want to do with me? I pledged allegiance on the blessed hand of the Messenger of Allah (peace and blessings be upon him) with this very hand of mine, and since then I have neither broken the covenant nor made any change (in my religion and faith)." (Hearing this) I drew my sword from its sheath and said: I will take your head to the amir. He said (with great steadfastness): "Then take it (cut off my head)." I asked: What has prompted you to this conduct? He said: The Messenger of Allah (peace and blessings be upon him) took a covenant from me and said: "When you see people pledging allegiance to two amirs, then take that sword of yours with which you fought alongside me and strike it against (Mount) Uhud until it breaks; then sit in your house until a sinful hand comes to you (to kill you) or a decreed death overtakes you."
I (Ibn Hajar) say: Imam Ahmad (may Allah have mercy on him) narrated through his chain another hadith with this meaning, but its context is not this one, nor does it contain the words "until a hand comes to you" to the end of the hadith; moreover, this chain is weak because it contains a narrator whose condition is not known.