Tafsir of Al-Mu'minoon 23:26

(He said) This is an explanatory initiation, as if it were asked: What did he—peace be upon him—say after he heard these falsehoods from them? It is said: He spoke when he saw they had persisted in what they were upon and continued in their misguidance until he despaired of their faith entirely, and it had been revealed to him: "That none will believe from your people except those who have already believed."

(My Lord, help me) by destroying them completely. This is based on the premise that this is a summarized account of his saying—peace be upon him—: "My Lord, do not leave upon the earth any of the disbelievers as an inhabitant," and so on.

The ba (ب) in His saying (بِمَا كَذَّبُونِ - bima kadhabun) is for causality or substitution. The ma (مَا) is a verbal noun (masdariyyah), meaning "because of their denying me" or "in place of their denial." It is also permitted that the ba is instrumental and the ma is relative (mawsulah), meaning "Help me by means of that with which they denied me," which is the punishment I promised them within my saying: "Indeed, I fear for you the punishment of a tremendous day." The essence of this is: "Help me by fulfilling that." The ambiguity in the omission of such a referent in the discourse is not hidden.

Abu Ja’far and Ibn Muḥayṣin read "Rabbi" (ربُّ) with a dammah on the ba. The basis for this is not hidden.