Musnad Ahmad 30:1159

'Ali ibn Hafs narrated to us, Waraqa narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Our stopping place tomorrow, if Allah wills—when Allah grants the conquest—is al-Khayf, where they swore an oath upon disbelief."