Musnad Ahmad 132:20

Hassan narrated to us, Ibn Lahi'ah narrated to us, Zabban narrated to us, from Sahl, from his father, from the Messenger of Allah (peace and blessings of Allah be upon him): "The Ummah will remain upon the Sharia as long as three things do not appear among them: as long as knowledge is not taken away from them, and children of adultery (walad al-hirth) do not become numerous among them, and the Saqqarun do not appear among them." He said: "And what are the Saqqarun — or the Saqlawun — O Messenger of Allah?" He said: "People who will be in the end of time, whose greeting among themselves will be cursing one another."