The oldest extant biography of Muhammad is called the "Sirat Rasulallah" â "Life of the Prophet of Allah". This book was written by Ibn Ishaq, a devout Muslim scholar, and later revised by Ibn Hisham. It was written before any of the major works of Hadith. It is considered the most authentic biography of Muhammad. It was translated into English by A. Guillaume as "The Life of Muhammad".
page 515 reads:
"Kinana al-Rabi, who had the custody of the treasure of Banu Nadir, was brought to the apostle who asked him about it. He denied that he knew where it was. A Jew came (Tabari says "was brought"), to the apostle and said that he had seen Kinana going round a certain ruin every morning early. When the apostle said to Kinana, "Do you know that if we find you have it I shall kill you?" He said "Yes". The apostle gave orders that the ruin was to be excavated and some of the treasure was found. When he asked him about the rest he refused to produce it, so the apostle gave orders to al-Zubayr Al-Awwam, "Torture him until you extract what he has." So he kindled a fire with flint and steel on his chest until he was nearly dead. Then the apostle delivered him to Muhammad b. Maslama and he struck off his head, in revenge for his brother Mahmud."