Khabaab (Radiallaho Anho) is one of those fortunate persons who offered themselves for sacrifice and suffering for the cause of Allah (Subhanahu wa Taala). He was the sixth or seventh person to embrace Islam and therefore, he suffered long. He was made to put on steel armor and lie in the sun to sweat. Very often he was made to lie flat on burning sand, which caused the flesh on his back to dissolve. He was the slave of a woman. When she came to know that he was visiting Rasulullah (Sallallaho Alaihi Wassallam), she used to brand his head with a hot iron rod. Umar (Radiallaho Anho) during his Caliph once asked him about the details of his suffering after embracing Islam.
He showed him his back, seeing which Umar (Radiallaho Anho) said, "I have never seen such a back before."
He said, "My body was dragged over heaps of burning coal and the blood and fat coming out of my back put out the fire."
When Islam spread and the Muslims conquered all the surrounding territory, he used to weep and say, "Allah (Subhanahu wa Taala) seems to be compensating us in this world for all our sufferings and perhaps nothing would be left for us as reward in the Akhirah".
Source: From the Book "Stories of the Sahabah" by Shaikh Muhammad Zakariyya Kandhalvi.