Surah 28. Al-Qasas, Ayah 44

Implying that the story of Moses as narrated in the Qur'an could not have come to Muhammad's knowledge otherwise than through revelation: consequently, the Qur'an as such must obviously be a result of divine revelation. - The term al-amr, rendered above as "the Law", is the Arabic equivalent of the Hebrew word torah ("law" or "precept"), the commonly-accepted title of the revelation granted to Moses.

 The pagans are reminded repeatedly in the Quran that the Prophet (ﷺ) did not witness any of these events that happened centuries before he was born. For example, the conspiracies that were made against Joseph (12:102), the dispute on who should be the guardian of young Mary (3:44), and the drowning of Noah’s son in the Flood (11:49). These details were not known to Arabs before the revelation of the Quran. So the only logical way that the Prophet (ﷺ) knew about these stories is through divine revelation.

The Sinai Peninsula is in the north-west corner of Arabia. But the reference here is, I think, to the western side of the valley of Tuwa. Mount Tur, where Moses received his prophetic commission, is on the western side of the valley.