Sulayman bin Buraydah (may Allah have mercy on him) narrates from his father, Sayyiduna Buraydah (may Allah be pleased with him), that whenever the Messenger of Allah (peace and blessings be upon him) sent someone off as the commander of an army, he would give him some instructions. This narration has the addition: "When you besiege a fortress and they ask you for the covenant of protection of Allah and His Messenger (peace and blessings be upon him), then instead of the covenant of Allah and the Messenger (peace and blessings be upon him), give them your own covenant and that of your companions; for if you break your own covenant and that of your fathers and companions, it is a lesser matter than breaking that of Allah and His Messenger (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18632