Why believe Muhammad was a prophet?

What the Quran says

"A lifetime before it" (10:16). "Say, 'If Allah had willed, I would not have recited it to you, nor would He have made it known to you, for I had remained among you a lifetime before it. Then will you not reason?'" (10:16), Meccan, addressed to the Quraysh. The argument: they had known him for forty years; nothing in his past prepared them for the Quran.

The challenge (2:23; 17:88). "And if you are in doubt about what We have sent down upon Our Servant [Muhammad], then produce a surah the like thereof and call upon your witnesses other than Allah, if you should be truthful" (2:23), Medinan, addressed to those who doubt. Elsewhere: "Say, 'If mankind and the jinn gathered in order to produce the like of this Qur'an, they could not produce the like of it…'" (17:88). The challenge is to produce something of the same kind. In al-Baqillani's classical account of the Quran's inimitability (iʿjāz), it lies chiefly in its unique composition (naẓm), alongside its reports of the unseen and the Prophet's (pbuh) being ummī (Divlekci 2019; see "Are 'scientific miracles' in the Quran real?").

The Byzantine prediction (30:2–4). "The Byzantines have been defeated In the nearest land. But they, after their defeat, will overcome. Within three to nine years" (30:2–4), Meccan. "Three to nine years" is Saheeh's rendering of biḍʿi sinīn, which Saʿid ibn Jubayr explained as "less than ten" (Ibn Kathir). Ibn Kathir says the verses "were revealed about the victory of Sabur, the king of Persia, over Ash-Sham", and records a wager: Abu Bakr (ra) bet the idolaters that Rome would win; they agreed on six years; the Romans won "when the seventh year came" (Ibn Kathir's abridgement: "This is a Hasan Hadith"). On sunnah.com the report, narrated by Niyar b. Mukram al-Aslami, is Tirmidhī 3194 Ḍaʿīf (weak) — al-Albānī; Ḥasan (good) — Aḥmad Shākir; Ḥasan ṣaḥīḥ — Bashshār ʿAwwād Maʿrūf; Good chain — Zubair ʿAlī Zaʾī; al-Tirmidhi calls it ḥasan ṣaḥīḥ gharīb, known only through ʿAbd al-Rahman b. Abi al-Zinad, and Darussalam grades it ṣaḥīḥ. A report from Ibn ʿAbbas in al-Tirmidhi (graded ḥasan gharīb) gives five years instead of six.

What is complicated about it. First, the dates: the Persians took Jerusalem "following a three weeks siege in 614"; Heraclius counter-attacked from 622 to 626, defeated the Persians at Nineveh on 12 December 627, and the war ended about 628 (Wikipedia, "Byzantine–Sasanian War of 602–628", read by the independent source check). The Ibn Kathir page read does not give dates. Second, an old variant reading vowels the verbs the other way — the Byzantines "have overcome… and will be defeated" — reported in the tafsir literature; the Ibn Kathir page read here does not mention it, so it is noted as unverified. A fair reader will say: the prediction was made in Mecca, at a time when Rome looked beaten; it came true; and the reports about it come from Muslim sources.