Praise be to Allah.
There are many ahaadeeth which indicate that a man’s ‘awrah is the area between the navel and the knee (and the navel and knee are not part of the ‘awrah). See al-Majmoo’, 3/173; al-Mughni, 2/286
These ahaadeeth include the following:
1 – Abu Dawood (3140) and Ibn Maajah (1460) narrated that ‘Ali (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Do not show your thigh, and do not look at the thigh of anyone, living or dead.”
2 – Ahmad (21989) narrated that Muhammad ibn Jahsh (may Allaah be pleased with him) said: The Prophet (peace and blessings of Allaah be upon him) passed by Ma’mar when I was with him, and his thighs were uncovered. He said: “O Ma’mar, cover your thighs, for the thigh is ‘awrah.”
3 – Ahmad (15502), Abu Dawood (4014) and al-Tirmidhi (2798) narrated from Jawhad al-Aslami that the Prophet (peace and blessings of Allaah be upon him) passed by him when his thigh was uncovered and he said: “Do you not know that the thigh is ‘awrah?”
4 – al-Tirmidhi (2798) narrated from Ibn ‘Abbaas (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “The thigh is ‘awrah.”
Shaykh al-Albaani said concerning these ahaadeeth in al-Irwa’(1/297):
Although their isnaads are not entirely free of weakness, they strengthen one another, because there is no narrator among them who may be regarded as suspicious, rather their weakness has to do with confusion and ambiguity. And there are similar reports which make one confident that the hadeeth is saheeh, especially since some of them were classed as saheeh by al-Haakim, and al-Dhahabi agreed with him. And al-Bukhaari narrated it in a mu’allaq report in his Saheeh… But a number of these isnaads give the hadeeth strength, and raise it to the level of being saheeh, especially sincere there are similar, corroborating reports on the same issue. End quote.
It says in Fataawa al-Lajnah al-Daa’imah (6/165):
Even though there is some weakness in the isnaads of these ahaadeeth, as there are gaps in the chain of narrators or there is some weakness in some of the narrators, nevertheless they support one another, and when taken as a whole they provide the required evidence. End quote.
The majority of fuqaha’ are of the view that these ahaadeeth should be followed and they stated that a man’s ‘awrah is from the navel to the knee. See al-Mughni, 2/284
And Allaah knows best.