Tafsir of Ash-Shu`ara' 26:31

(He said) as he hoped to find a basis for opposition: (Bring it) that is, bring something manifest (if you are among the truthful), meaning: in what your words indicate, such that you bring something clarifying the truth of your claim, or truthful in the claim of the message from the Lord of the worlds. The response to the conditional clause is omitted because what precedes it serves as an indication of it; that is: "If you are among the truthful, then bring it." Al-Zamakhshari estimated it as: "Then you have brought it." The well-known estimation is that it is of the same nature as the proof.

Al-Hufi said: It is permissible for what precedes [the conditional clause] to be the answer, and it is permissible for the answer to be advanced because the omission of the condition did not have a grammatical effect on the wording. Al-Zamakhshari—may Allah the Exalted deal with him according to His justice—has slandered the people of Sunnah with things of which they are innocent, as explained by the author of al-Kashf and others, so refer to it if you wish.