What is the evidence for the obligation of covering one's 'awrah during prayer, and why do scholars obligate covering the entire body even though the hadiths only mention the khimar? And what is the reason for adopting the weak hadith from Al-Muwatta' while abandoning other hadiths due to the same defect?

The jurists have unanimously agreed on the obligation of covering the 'awrah (private parts) during prayer. However, they differed on whether it is a condition for the validity of the prayer or merely an obligation. They cited the Almighty's saying: "O Children of Adam, take your adornment at every masjid (place of prayer)." Some scholars have argued that the verse indicates that what is obligatory in prayer is more than merely covering the 'awrah, basing their argument on the Prophet's prohibition of a man praying in a single garment with nothing on his shoulder, his prohibition of circumambulating unclothed, and the obligation of the head covering (khimar) for a free woman during prayer, even though she is permitted to remove it in front of her mahrams (unmarriageable kin).