Musannaf Ibn Abi Shayba 36:471

Abdullah ibn al-Mubarak narrated to us, from Ma'mar, from al-Zuhri, from Abd al-Rahman ibn Ka'b, from his father, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) intended a military expedition, he would imply another one. Until it was the expedition of Tabuk; the Messenger of Allah (peace and blessings of Allah be upon him) traveled in intense heat and faced a long journey. So, he clarified the matter to the Muslims and informed them of that, so that they might prepare the preparation for their enemy, and he informed them of the direction he intended.