Mishkat al-Masabih 3938
Ka‘b b. Malik said:
When God’s Messenger intended to go on an expedition he always pretended to be going somewhere else till that expedition, meaning the one to Tabuk. God’s Messenger undertook it in extreme heat, facing a long journey, desert country and a numerous enemy. He made clear to the Muslims what they were about to do in order that they might get ready the equipment for their expedition, telling them where he was going. 

Bukhari transmitted it.
وَعَن كَعْب بن مالكٍ قَالَ: لَمْ يَكُنْ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُرِيدُ غَزْوَةً إِلَّا وَرَّى بِغَيْرِهَا حَتَّى كَانَتْ تِلْكَ الْغَزْوَةُ يَعْنِي غَزْوَةَ تَبُوكَ غَزَاهَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي حَرٍّ شَدِيدٍ وَاسْتَقْبَلَ سَفَرًا بَعِيدًا وَمَفَازًا وَعَدُوًّا كَثِيرًا فَجَلَّى لِلْمُسْلِمِينَ أَمْرَهُمْ لِيَتَأَهَّبُوا أُهْبَةَ غَزْوِهِمْ فَأَخْبَرَهُمْ بِوَجْهِهِ الَّذِي يُرِيدُ. رَوَاهُ الْبُخَارِيُّ