Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.111.

Says Allah in the Qur’an, “O Children of Adam, take your adornment (by wearing proper clothing) for every mosque” (al-A’raf 31). The meaning of “adornment” here is the covering of the ‘aurah. The meaning of “mosque” is “prayer.” Therefore, it means “Cover your ‘aurah for every prayer.” Salamah ibn al-Aku’ said to the Prophet, “O Messenger of Allah, may I pray in a long shirt?” He said, “Yes, but button it, even with just a thorn.” (Related by al-Bukhari in his Tareekh.)