Sulayman bin Buraydah narrates from his father (may Allah be pleased with him) that whenever the Master of both worlds (peace and blessings be upon him) appointed a commander over a small detachment or a large army, he would advise him "to fear Allah Almighty in his own affairs and to treat the Muslims with him well."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18633