يَا أَيُّهَا الَّذِينَ آمَنُوا قَاتِلُوا الَّذِينَ يَلُونَكُم مِّنَ الْكُفَّارِ وَلْيَجِدُوا فِيكُمْ غِلْظَةً ۚ وَاعْلَمُوا أَنَّ اللَّهَ مَعَ الْمُتَّقِينَ
“O you who have believed, fight those surrounding you of the disbelievers and let them find in you harshness. And know that Allah is with the righteous.”
Cited by al-Tabari as the command to fight the neighboring Arabs before turning north, read as the trigger for the expedition.