Praying in a light thobe

If the thobe [men’s garment, a long white robe as worn in Saudi Arabia and other Arab countries] is so thin that the colour of the skin beneath it can be seen, then you cannot pray wearing it, because it does not cover the ‘awrah properly. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) was asked about that and said: 

If the thobe in question does not conceal the skin because it is thin or light, then a man cannot pray in it unless he is wearing trousers or an izaar [lower garment like a sarong] underneath it to cover the area between the navel and the knees). 

A woman cannot pray in light or thin clothes unless she is wearing something underneath that covers the entire body. 

Short pants underneath the thobe in question are not sufficient. 

If a man prays in such a thobe he should also wear a undershirt or something else to cover the shoulders, or one of the them, because the Prophet (peace and blessings of Allah be upon him) said: “No one of you should pray in a single garment with nothing on his shoulders.” (Saheeh – agreed upon). 

Fatawa Islamiyyah, 1/219 

But if the thobe covers the skin and shows nothing of its colour, but it is so thin or tight that it shows the size of the limbs, this does not affect the prayer, so long as the ‘awrah is covered. 

Ibn Qudaamah said: It is obligatory to cover with something that conceals the colour of the skin; if it is so light that the colour of the skin can be seen through it, so that it can be seen if the skin is white or red, then it is not permissible to pray in it, because that does not cover properly. If it conceals the colour but shows the shape, it is permissible to pray in it, because this cannot be avoided.  

Al-Mughni, 2/286.