The Quran Verse
Surah 13:7
And those who disbelieved say, 'Why has a sign not been sent down to him from his Lord?' You are only a warner, and for every people is a guide.
Throughout Surah 13, the Meccans challenge Muhammad to perform a physical miracle like those of Moses (parting the sea) or Jesus (healing the blind). The Quranic response is consistently a redirection: "The Quran is enough," or "Miracles are with God."
Critics argue that if Muhammad were truly in the line of the great prophets, he should have possessed the same confirming signs. Surah 13:31 even admits that even if a miracle moved mountains or made the dead speak, the disbelievers still wouldn't believe.
This sounds like an after-the-fact excuse for the lack of supernatural evidence to back up his claims.