The Night Prayer  in Ramadaan is a cause for attaining the forgiveness of sins. The Prophet, (صلى الله علیه وسلم) said:
“Whoever stands (in night prayer) during Ramadaan with faith and seeking reward (from Allaah), all of his previous sins will be forgiven.” [Al-Bukhaaree and Muslim]
‘Aa’ishah (رضي الله عنها) clarified the number of rak’aat for the Night Prayer, as the Prophet, (صلى الله علیه وسلم) used to pray it.
“The Prophet, (صلى الله علیه وسلم) did not exceed eleven (11) rak’aat in Ramadaan or out of it.” [Reported by Al-Bukhaaree and Muslim] 
“When the last ten days of Ramadaan would come, the Prophet, would tighten his waist-wrapper (izaar), spend the night in worship, and wake his family.” [Reported by Al-Bukhaaree and Muslim]
“Allaah’s Messenger, (صلى الله علیه وسلم) would exert himself in the last ten days (of Ramadaan) more than he would at other times.” [Reported by Muslim]
“The Messenger of Allaah, (صلى الله علیه وسلم) would perform ‘Itikaaf (seclusion in the masjid) during the last ten days of Ramadaan.” [Reported by Al-Bukhaaree and Muslim]
“When the Prophet, (صلى الله علیه وسلم) wanted to perform ‘Itikaaf, he would pray Fajr (in the masjid) and then begin his ‘Itikaaf.”  [Reported by Al-Bukhaaree and Muslim]
One of the most important reasons why the last ten nights are so honorable is because Laylat-ul-Qadr (The Night of Decree) falls in it. Allaah says:
“Laylat-ul-Qadr is better than a thousand months.” [Surah Al-Qadr: 3]
And He says:
“Indeed, We revealed it (this Qur’aan) on a blessed night.” [Surah Ad-Dukhaan: 3]
The Prophet, (صلى الله علیه وسلم) said:
“Whoever stands in night prayer during Laylat-ul-Qadr with faith and seeking reward (from Allaah), his past sins will be forgiven.” [Reported by Al-Bukhaaree and Muslim]
And he, (صلى الله علیه وسلم) said:
“On the morning following Laylat-ul-Qadr, the sun rises not having any rays.”[/i]  [Reported by Muslim]
This was the sign one of the Companions  used to determine that Laylat-ul-Qadr was the twenty-seventh night of Ramadaan and Allaah knows best.
 Commonly known as Salaat-ut-Taraweeh.
 The report that the Prophet (صلى الله علیه وسلم) prayed twenty rak'aat is not authentic, nor the report that 'Umar (radi-Allaahu anhu) did it.
 Based on this, the start of 'Itikaaf should begin on the morning of the twentieth day, immediately after Salaat-ul-Fajr. This is because Al-Bukhaaree reported a hadeeth that indicates that one should leave 'Itikaaf in the morning also – meaning the morning of the thirtieth day. And he (Al-Bukhaaree) named this chapter: "The one who leaves his 'Itikaaf during the morning."
 It appears round like the moon.
 He is Ubay Bin Ka'ab (رضي الله عنه).