In ancient Greece the "Sacred Band" referred to a group of 300 elite warriors from which Greek city-state?

The Sacred Band of Thebes was a troop of hand picked soldiers, consisting of 150 pairs of male lovers which formed the elite force of the Theban army in the 4th century BC. Believed to have been formed by Gorgidas in 378 BC, they gained their fierce reputation in the battle of Tegyra in 375 BC, when the Thebans defeated a Spartan army that was three times their size. In 371 BC they played a crucial role in the defeat of the Spartans again at the battle of Leuctra, leaving Thebes as the most powerful city-state at that time. The demise of the Sacred Band of Thebes came in 338 BC at the battle of Chaeronea, where they were all but annihilated by the Macedonian forces of Phillip II, most noticeably those under the command of Phillip's son Alexander.

Around 300 BC the city of Thebes had a monument erected at the burial site at Chaeronea which still stands today.

Not to be mistaken with the 300 Spartans who famously died at Thermopylae. They were the personal guard of Leonidus and were never known as "The Sacred Band".