Musnad Abu Ya'la al-Mawsili 130:29

Ja'far ibn Mihran narrated to us, 'Abd al-Warith narrated to us, from 'Abd al-'Aziz ibn Suhayb, from Anas ibn Malik, who said: The Messenger of Allah (ﷺ) sent seventy men for a need, who were called "the Qurra" (the reciters). Two tribes from Banu Sulaym, Ri'l and Dhakwan, confronted them at a well called Bi'r Ma'unah. The people said, "By Allah, we did not intend [to fight] you; we are only travelers on a mission for the Messenger of Allah (ﷺ)." But they killed them. The Messenger of Allah (ﷺ) supplicated against them for a month during the morning prayer. That was the beginning of the Qunut, and we did not use to perform Qunut [before that].