Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) says that whenever the Prophet (peace and blessings be upon him) dispatched an army of Muslims against the idolaters, he would say: "Set out 'Bismillah' (in the name of Allah)." He mentioned the hadith, in which it says: "Do not kill children, women or very old men. Do not block up any spring. Cut down only that tree which obstructs your fighting between you and the idolaters. Do not mutilate any human being or animal. And do not commit treachery or steal from the spoils either."
There is irsal (a break in the chain) and weakness in this chain, but it is strengthened by its supporting reports together with the narrations (athar) on this subject. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18155