The Battle of Uhud
Quraysh marched out three thousand strong to avenge Badr, and the Muslims met them by Mount Uhud. Victory was theirs at first, until the archers disobeyed the Prophet's ﷺ order; the tide turned and seventy were martyred, among them Hamza, the Master of the Martyrs.
Quraysh came with their followers driven by the vengeance of Badr, so the Prophet ﷺ went out with a thousand, a third of whom turned back with the head of hypocrisy, Ibn Ubayy. He stationed the fifty archers on the hill and ordered them not to leave whether the Muslims won or lost.
The polytheists were routed at the start of the day, but when the archers saw the spoils, most of them came down. Khalid ibn al-Walid — a polytheist that day — wheeled around from behind the hill, the ranks were thrown into disorder, the Prophet's ﷺ face was gashed and his tooth broken, and Satan cried: Muhammad is killed — it was a grievous hour.
Seventy were martyred, among them Hamza ibn ʿAbd al-Muttalib and Musʿab ibn ʿUmayr, the banner-bearer. He ﷺ held firm until he drew back to the mountain, and verses of Al ʿImran were revealed that day, admonishing and nurturing: "And Allah had certainly fulfilled His promise to you... and that Allah might purify those who believe." So Uhud was the Ummah's school in the cost of disobedience and the etiquette of trial.
Note — differing reports on the date: The well-known view is Saturday 7 Shawwal 3 AH; some say the middle of Shawwal.