Musnad Ahmad 31:820

Khalf ibn al-Walid narrated to us, 'Abbad ibn 'Abbad narrated to us, from Mujalid ibn Sa'id, from Abu al-Wadak, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Mudar [tribe] of the servants of Allah shall certainly be struck, until the name of Allah is not worshipped. And the believers shall certainly strike them, until they do not withhold [even] the tail of a hill."