Musnad Ahmad 30:1155

Abu Abd al-Rahman narrated to us, Sa'id narrated to us, Muhammad ibn 'Ajlan narrated to us, from al-Qa'qa' ibn Hakim, from Abu Salih, from Abu Hurayrah, from the Messenger of Allah, peace and blessings of Allah be upon him, that he said: "There will not cease to be for this matter—or upon this matter—a group upon the truth. Those who oppose them will not harm them, until the command of Allah comes to them."