إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِيْنَ

Inna idhaa nazalnaa bisaahati qawm, fasaa’a sabaahul mundharin.

Surely when we approach a (hostile) nation (to fight), then evil will be the morning for those who have been warned."

Reference: Bukhari: 4197