Tafsir of Al-Anbiya' 21:68

They reached a consensus to destroy him once they were defeated. This is the way of the falsifier: when his doubt is struck by proof and he is exposed, nothing is more hateful to him than the person of truth. He has no refuge left but to openly oppose him, just as the Quraysh did with the Messenger of Allah (ﷺ) when they were unable to counter his arguments.

The one who suggested burning him was Nimrod. Ibn Umar (may Allah be pleased with them) said it was a non-Arab Bedouin, meaning a Kurd.

It is narrated that when they resolved to burn him, they imprisoned him and built a structure like an enclosure in Kutha. For a month, they gathered all kinds of hard wood. Even a woman, if she fell ill, would vow: "If Allah heals me, I will gather firewood for Ibrahim (peace be upon him)." Then they ignited a fire so great that birds in the sky were nearly scorched by its heat. They placed him in a catapult, bound and shackled, and cast him into it. Gabriel (peace be upon him) called out to it: "We said, 'O fire, be coolness and safety upon Ibrahim.'"

It is said that it burned nothing but his bonds. When he was thrown, Gabriel asked him, "Do you have any need?" He replied, "As for you, no." Gabriel said, "Then ask your Lord." He replied, "His knowledge of my state is sufficient for me without my asking." Ibn Abbas (may Allah be pleased with them) said: He was saved only by saying, "Allah is sufficient for me, and He is the best Disposer of affairs."

Nimrod looked down from his palace and saw him in a garden with an angel sitting beside him. Nimrod said, "I will draw near to your God," and he sacrificed four thousand cows and left Ibrahim alone. Ibrahim (peace be upon him) was sixteen years old at the time.

They chose punishment by fire because it is the most terrifying and horrific form of punishment. Hence the saying: "None punishes with fire except the Creator of it."

Their words, "If you are to act," mean: If you are to support your gods with a decisive victory, then choose for him the most terrifying punishment—burning—otherwise, you have failed to support them. For this reason, they magnified the fire, went to great lengths to publicize it, and spared no effort in doing so.

The fire was made to comply with Allah’s command and will, like one commanded to do something who then obeys. The meaning is: "Be a source of coolness and safety," using hyperbole as if the very essence of the fire became coolness and safety. The intent is: "Cool down, so that Ibrahim may be safe from you," or "Cool down with a coolness that does not harm." Ibn Abbas said: "Had He not said that, it would have destroyed him with its cold."

If you ask: "How did the fire become cool while remaining fire?" I say: Allah stripped from it the nature of heat and burning that He had originally placed within it, while leaving its light and combustion as they were. Allah is capable of all things. It is also possible that He protected Ibrahim’s body from the harm of its heat by His power, allowing him to experience the opposite, just as He does for the keepers of Hell. This is indicated by the words: "upon Ibrahim."

They intended to plot against him and deceive him, but they were the ones defeated and subdued. They were defeated in debate, so Allah gave him the upper hand and taught him the silencing argument. They then resorted to force and tyranny, but He supported and strengthened him.