Narrated Ka'b bin Malik:
Whenever Allah's Apostle intended to carry out a Ghazwa, he would use an equivocation to conceal his real destination till it was the Ghazwa of Tabuk which Allah's Apostle carried out in very hot weather. As he was going to face a very long journey through a wasteland and was to meet and attack a large number of enemies. So, he made the situation clear to the Muslims so that they might prepare themselves accordingly and get ready to conquer their enemy. The Prophet informed them of the destination he was heading for (Ka'b bin Malik used to say, "Scarcely did Allah's Apostle set out for a journey on a day other than Thursday.")