Whenever the Messenger (may Allah's peace and blessings be upon him) intended to carry out a battle, he would use an equivocation to conceal his real destination till it was the battle of Tabūk
Authentic hadithعن كعب بن مالك رضي الله عنه قال: «كان رسول الله صلى الله عليه وسلم قَلَّمَا يُريد غَزْوَةً يَغْزُوها إلا وَرَّى بغيرها، حتى كانت غزوة تبوك، فَغَزَاهَا رسول الله صلى الله عليه وسلم في حَرٍّ شَديدٍ، واستقبل سَفَراً بعيداَ ومَفَازَا، واستقبل غَزْوَ عَدُوٍّ كثير، َفَجَلَّى للمسلمين أَمْرَهم، لِيِتَأَهَّبُوا أُهْبَةَ عَدُوِّهم، وأخبرهم بِوَجْهِهِ الذي يُريدُ».
Ka‘b ibn Mālik (may Allah be pleased with him) reported: Whenever the Messenger (may Allah's peace and blessings be upon him) intended to carry out a battle, he would use an equivocation to conceal his real destination till it was the battle of Tabūk which the Messenger 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 (may Allah's peace and blessings be upon him) informed them of the destination he was heading for.
Explanation
As related in this Hadīth, whenever the Prophet (may Allah's peace and blessings be upon him) set out for a battle, he would ask about a certain route other than the intended one so as to deceive the enemy and prevent spies from reporting their news. He wanted to reach the enemy while they were unprepared for the confrontation. An exception was the battle of Tabūk. As it would entail a long and arduous journey, he did not conceal the destination. And Allah, the Almighty, granted Muslims victory over the disbelievers.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).