Tafsir of Ash-Shu`ara' 26:27

[He said], exaggerating in the response and signaling a lack of regard for it, explicitly stating what would alienate their hearts from the speaker and from accepting what he brought: "Indeed, your messenger who has been sent to you is mad."

This is because he is asked about one thing and answers with another, and he is alerted to what is in his answer, yet he does not take heed. He called him a "messenger" by way of mockery, attributing him to his addressees to elevate himself above the possibility of such a person being sent to him. He emphasized this with the description, intending thereby to incite their anger and provoke them to deny his messengership after hearing the news, as they would deem themselves too lofty to be worthy of having a madman sent to them.

Mujahid and al-A’raj read it as "arsala" (active voice), meaning the One who sent him, his Lord, to you. It is as if he—peace be upon him—upon seeing the harshness of the accursed one's response, realized that the cursed one had signaled that he—peace be upon him—had not grasped the obscurity contained within his first answer to his people. His deviation from it to the second answer was due to what the accursed one had accused him of.