Can you please specify a man's 'Awrah', providing Sahih Hadith or Ayah from the Qur'an? Can a Muslim man pray wearing only underwear? Is there a proof from the Sunnah or the Qur'an that forbids a man (not a woman) to wear tight clothes?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The majority of Muslim scholars define that the limits of the 'Awrah of a man is what is between the navel and the knee; the navel and knee are not from the ‘Awrah. There are many proofs supporting their stand such as the Hadeeth Imams Ahmad, Abu Daawood, and Ad-Daaraqutni reported with a good chain that the Prophet said: “What is between the navel and the knee is ‘Awrah.”

Besides, ‘Ali narrated that the Prophet said: “Do not unveil your thigh, and do not look at the thigh of the living and the dead.” [Abu Daawood and Ibn Maajah]

Also, Ibn Abbas narrated that the Prophet once passed by a man whose thigh was uncovered. He said to him: “Cover your thigh, indeed thigh is (a part) of ‘Awrah.” [Ahmad and At-Tirmithi]

On the other hand, Jarhad Al-Aslami narrated that the Prophet passed by him while his [Jarhad’s] thigh was uncovered, so he said to him: “Cover your thigh, for it is (a part) of the Awrah.” [Maalik, Ahmad, Abu Daawood and At-Tirmithi]

Therefore, one should not wear tight clothes that expose the shape of his ‘Awrah. This act is contrary to the good manners and the dress of the righteous predecessors as well. The ‘Awrah should be totally covered while the tight dress exposes its shape and violates its covering.

For more benefit on praying in underwear, please refer to Fatwa 81473.

Allah knows best.