What is the ruling on praying in a garment that is slightly open at the neck, and is it obligatory to wear a garment that clings slightly to the body so that nothing shows during bowing (rukuʿ)?

A man's 'awrah in prayer is the area between the navel and the knee. Anything other than that is not considered 'awrah. Therefore, there is no objection to clothing that reveals a part of the neck or the upper chest, unless the 'awrah can be seen through a wide opening in the shirt. In that case, the prayer becomes invalid due to the failure of one of its conditions. This applies if it is possible for it to be seen, even if it is not actually seen due to blindness or darkness. If the shirt is not buttoned but is wide, and he tightens it around his waist with something that covers the 'awrah, or if his beard covers the opening, his prayer is valid.