Musnad Ahmad 32:1518

Ubaydah ibn Humayd narrated to us, from Humayd al-Tawil, from Anas ibn Malik, who said: "There were seventy young men from the Ansar who were called 'the reciters' (al-Qurra')." He said: "They used to be in the mosque. When evening came, they would withdraw to a side of Medina, where they would study together and pray. Their families would think they were in the mosque, and the people of the mosque would think they were with their families. Until, when it was near dawn, they would fetch fresh water and gather firewood, then bring it and lean it against the chamber of the Messenger of Allah, peace and blessings of Allah be upon him. The Prophet, peace and blessings of Allah be upon him, sent them all out, and they were struck down on the day of Bi'r Ma'una. The Prophet, peace and blessings of Allah be upon him, invoked curses upon their killers for fifteen days during the morning prayer."