When Al Mamoun was still in Khorassan, a revolt was raised against him in Bagdad by his uncle, Ibrahim, the son of Mahdi. This prince had great talent as a singer, and was a skilful performer on musical instruments. Being of a dark complexion, which he inherited from his mother, Shikla, who was a negress, and of a large frame of body, he received the name of At-Tinnin (the Dragon). He was proclaimed Caliph at Bagdad during the absence of Al Mamoun. The cause which led the people to renounce Al Mamoun and choose Ibrahim was that the former had chosen as his successor one of the descendants of Ali, and in doing so had ordered the public to cease wearing black, which was the distinctive color of the Abbassides, the reigning family, and to put on green, the color of the family of Ali and their partizans. On Mamoun's entry into Bagdad, Ibrahim fled disguised as a woman. He was, however, detected and arrested by one of the negro police. When he was before Al Mamoun, who addressed him in ironic terms, he replied: "Prince of the believers, my crime gives you the right of retaliation, but 'forgiveness is near neighbor to piety.'[9] God has placed you above all those who are generous, as he has placed me above all criminals in the magnitude of my crime. If you punish me you will be just; if you pardon me you will be great." "Then I pardon you," said Mamoun, and prostrated himself in prayer. He commanded, however, that Ibrahim should continue to wear the _burqa_, or long female veil in which he had fled, so that people might see in what disguise he had been arrested; he ordered also that he should be exposed to view in the palace courtyard; then he committed him to police supervision, and finally, after some days of detention, set him free. The following anecdote was related by Ibrahim regarding the time when he was in hiding with a price set on his head: "I went out one day at the hour of noon without knowing whither I was going. I found myself in a narrow street, which ended in a _cul-de-sac_, and noticed a negro standing in front of the door of a house. I went straight to him, and asked if he could afford me shelter for a short time. He consented, and bade me enter. The hall was adorned with mats and leather cushions. Then he left me alone, closed the door, and departed. A suspicion flashed across my mind; this man knew that a price was set on my head, and had gone to denounce me.