Musnad Ahmad 175:19

'Attab ibn Ziyad narrated to us, he said: 'Abdullah narrated to us, he said: Yunus informed us, from al-Zuhri, he said: 'Abd al-Rahman ibn 'Abdullah ibn Ka'b informed me, that 'Abdullah ibn Ka'b said: I heard Ka'b ibn Malik saying: "The Messenger of Allah, peace and blessings of Allah be upon him, would rarely intend a military expedition that he would fight, except that he would allude to other than it, until the expedition of Tabuk occurred. The Messenger of Allah, peace and blessings of Allah be upon him, fought it in intense heat, facing a long journey and a desert, and facing the fighting of a numerous enemy. So he clarified their affair to the Muslims so that they might prepare the preparation for their enemy; he informed them of the direction he intended."