Jami` at-Tirmidhi 511

Iyad bin Abdullah bin Abi Sarh narrated:"Abu Sa'eed Al-Khudri entered (the mosque) on Friday while Marwan was giving the Khutbah, so he began praying. Two guards came to make him sit down but he refused until he had prayed. When he finished he came to us and we said: 'May Allah have mercy upon you. They nearly harmed you.' He said: 'I was not going to stop performing them (the two Rak'ah) after what I saw from Allah's Messenger.' Then he mentioned that a man who appeared untidy came on Fridy while the Prophet was delivering the Friday Khutbah, so he ordered him to pray two Rak'ah all the while the Prophet was delivering the Khutbah