The Messenger of Allah peace be upon him sat with us and my thigh was uncovered. He said: Do you not know that thigh is a private part?
First of all, about the narrator of the hadith, Jarhad.
Jarhad ibn Khuwaylid Al-Aslami, nickname Abu Abdurrahman. He was of the people of Suffa. He participated in Hudaybiyah. He narrated several famous hadiths from The Messenger of Allah may peace be upon him. One of these hadiths is that 'the thigh of people is awrah'.
Some of the hadiths collected by him are included in the books by the three Sahih owners.
Jarhad lived in Medina, where he died during Yazid bin Mu'awiya.
It is said in the hadith, "Why do you open your thighs and hide it?!" From this it is clear that the thigh is awrah and that it is necessary to reprove other people who have been exposed to it and other unlawful situations.