Mas’oodi says that when their speech concluded and Muslim spoke harshly to Ubaydullah, he ordered that Muslim should be taken to the roof of the palace and it was said to Bukayr bin Humran Ahmari to behead him and take his revenge.
Jazari says that Muslim (a.s) told Mohammad bin al-Aash’as, “By Allah! I would never have surrendered if you had not given me the promise of protection. Then defend me with your sword for your promise has been broken.” Then they took him on top of the palace when he was asking forgiveness from Allah and praising and glorifying Him. Then they took him to the place overlooking the shoe-makers and severed his blessed head which fell down. (May Allah’s Mercy and Blessings be upon him). His murderer was Bukayr bin Humran, whom Muslim had previously wounded. Then his body too was thrown down. When Bukayr came down, Ubaydullah asked him, “What was Muslim uttering when you took him to the roof”?. He relied that, “Muslim was glorifying Allah and seeking His forgiveness. When I intented to kill him, I told him to come near and then I said: Praise be to Allah who has given me an upper hand over you and thus I have taken the revenge from you. Then I struck a blow, which went waste. Then Muslim said: O slave! Haven’t you taken your revenge by inflicting this wound upon me”? Ubaydullah said, “Such dignity even at the verge of death”? Bukayr said, “Then I struck him a second blow and killed him.”
Tabari says that Muslim was taken on the roof of the palace and his neck was severed and body thrown down to the people. An order was issued that his corpse be taken to that place where garbage is thrown and to be hanged there.
source : http://www.maaref-foundation.com