The one who prayed badly" (radi Allaahu `anhu)
In Hadeeth and Fiqh literature, this term refers to the Companion mentioned in the following hadeeth of Saheeh al-Bukhaari (Book of Prayer, English translation by Dr. Muhammad Muhsin Khan); many other narrations of this incident are found in the various collections of hadeeth, and provide an important source of instructions from the Prophet (sallallaahu `alaihi wa sallam) regarding the correct way to pray:
Narrated Abu Hurairah (radi Allaahu `anhu):
The Messenger of Allaah (sallallaahu `alaihi wa sallam) entered the mosque and a person followed him. The man prayed and then went to the Prophet (sallallaahu `alaihi wa sallam) and greeted him; he returned the greeting and said (to him), Go back and pray, for you have not prayed. The man went back and prayed in the same way as before, and then returned and greeted the Prophet (sallallaahu `alaihi wa sallam), who said, Go back and pray, for you have not prayed, three times. The man said, "By Him Who sent you with the Truth, I cannot do so any better than this, so please teach me." He said, When you stand for the Prayer, say takbeer and then recite what is easy for you from the Qur'aan (from what you know by heart); then bow until you feel at ease in rukoo`; then raise your head and stand up straight, then prostrate until you feel at ease in sajdah; then sit with calmness until you feel at ease, and do likewise in all your prayers.
[further narrations of this hadeeth found in the other works of Hadeeth such as Sunan Abi Daawood, etc. contain further details.]