Can't nail this one down, but according to Jean Strouse's excellent biography of Morgan, he'd given the inventor $150,000. in early 1901 to build a worldwide communications system, in return for a 51 % interest in his patents. Tesla built a 200 foot communications tower on Long Island, but by 1904 nothing was working and he was out of money. In an emotional letter, he asked for another $75,000 and the less-emotional Morgan replied, through his secretary, that he "could not do anything more in the matter". Nor did he fund any more Tesla projects ( though his son Jack did lend Tesla another $25,000 after J.P. died). Unfortunately, Strouse doesn't reference the source for her quote. It's possible that Morgan just said "no", and the secretary put it into more polite terms.

Jean Strouse: Morgan: American Financier

The letter is quoted in Richard Munson's Tesla: Inventor of the Modern, W. W. Norton & Company (2018), p.163 with an endnote referring to a letter from Tesla to JP Morgan dated October 17, 1904:

When he still heard nothing, Tesla lashed out and criticized the "Great Man", declaring, "You are no Christian at all, you are a fanatic musoulman [Muslim]. Once you say no, come what may, it is no."

It seems he used the term to accuse Morgan of inflexibility or hard-headedness, which in Tesla's mind were not Christian, but rather Muslim characteristics.