Musnad Abu Ya'la al-Mawsili 115:80

Abu Bakr narrated to us, Muhammad ibn Bishr narrated to us, from Sa'id, from Qatadah, from Anas ibn Malik: That 'Usayyah, Dhakwan, and Banu Lihyan came to the Prophet, may Allah bless him and grant him peace, and sought his aid, and they had claimed Islam against their enemy. So he aided them with seventy of the Ansar who were called "the reciters" (al-Qurra'). They used to gather firewood by day and pray by night. Until, when they were at the Well of Ma'unah, they killed them. So the Messenger of Allah, may Allah bless him and grant him peace, performed Qunut for a month, supplicating against them.