Guru Arjan Dev, the fifth of the Sikh Gurus, became the first martyr in the history of Sikhism. He attained the status of shaheed as one who sacrificed his life for the sake of his faith at the hands of the Islamic Mughal Empire. When ordered to change certain passages of scripture which offended Mughal rulers, he refused. His unwillingness to comply resulted in his being arrested and tortured to death during the hottest part of the year.
Source: The Events of Guru Arjun Dev Shaheedi (Martyrdom) | SikhNet