Muhammad bin Hamzah al-Aslami (may Allah be pleased with him) narrates from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) made him the commander of a detachment. He says: I went out with that detachment. He (peace and blessings be upon him) said: "If you find so-and-so, burn him with fire." As I was leaving, he (peace and blessings be upon him) called out to me. When I came back, he (peace and blessings be upon him) said: "If you find so-and-so, kill him, but do not burn him; for none may punish with fire except its Lord."
(b) The hadith of Usamah bin Zayd (may Allah be pleased with them both), in which the Messenger of Allah (peace and blessings be upon him) ordered him to burn Ubna (a settlement), and what is narrated about setting up the catapult against Ta'if, do not contradict what we have said. That concerns only fighting the polytheists while they are resisting (and beyond control), whereas the prohibition (of punishing with fire) concerning the polytheists applies when they are captives.
Imam al-Shafi'i (may Allah have mercy on him) likened this to shooting game while it is beyond control, and then the prohibition of shooting a hen that is not beyond control (that is, one that is tied up).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18065