Sahih Bukhari Volume 3, Book 32, Hadith Number 227.
Narated By Abu Huraira : Allah’s Apostle said, “Whoever prayed at night the whole month of Ramadan out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven.” Ibn Shihab (a sub-narrator) said, “Allah’s Apostle died and the people continued observing that (i.e. Nawafil offered individually, not in congregation), and it remained as it was during the Caliphate of Abu Bakr and in the early days of ‘Umar’s Caliphate.” ‘Abdur Rahman bin ‘Abdul Qari said, “I went out in the company of ‘Umar bin Al-Khattab one night in Ramadan to the mosque and found the people praying in different groups. A man praying alone or a man praying with a little group behind him. So, ‘Umar said, ‘In my opinion I would better collect these (people) under the leadership of one Qari (Reciter) (i.e. let them pray in congregation!)’. So, he made up his mind to congregate them behind Ubai bin Ka’b. Then on another night I went again in his company and the people were praying behind their reciter. On that, ‘Umar remarked, ‘What an excellent Bid’a (i.e. innovation in religion) this is; but the prayer which they do not perform, but sleep at its time is better than the one they are offering.’ He meant the prayer in the last part of the night. (In those days) people used to pray in the early part of the night.”