It is narrated from Sayyiduna Ka'b bin Malik (may Allah be pleased with him), (speaking of) when he stayed behind from the expedition, that whenever the Prophet (peace and blessings be upon him) intended to go on a military expedition, he would make an allusion (tawriyah) to some other destination.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13279