Salaah being the time of presenting oneself in the Divine Court of Allah Ta’ala requires a certain decorum and respect. As certain occasions demand a certain dress code, the Masjid also demands the same.
It is truly lamentable that we find individuals in many Masjids, going into Ruku and Sajdah and simultaneously exposing their rear end due to improper attire.
Firstly, the Islamic ruling is that if 1/8 of the area of the buttocks is exposed on account of carelessness and lack of concern, the Salaah will be invalidated instantly. If it is exposed involuntarily or mistakenly for the duration of reciting Subhaana Rabbiyal Azeem/Aa’laa three times, the Salaah will also be invalidated.
Secondly, the demand of the Masjid is proper attire. Revealing attire is not only disrespectful to the Masjid, it is disrespectful to the Musallees behind you to whom your rear is exposed.
At the very least we should keep a Kurta or Jubba in our cars for whenever we perform Salaah or use the Masjid Jubbas when performing Salaah at the Masjid. In this way the sanctity of the Masjid is preserved as well as the validity of our Salaah.
NB. This ruling applies to females as well, but is stricter in its application as a female’s back above the waistline must also be compulsorily covered.