Sahih Muslim Book - 6, Hadith - 1780

وَحَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ أَخْبَرَنَا عَبْدُ الرَّزَّاقِ أَخْبَرَنَا مَعْمَرٌ عَنِ الزُّهْرِىِّ عَنْ أَبِى سَلَمَةَ عَنْ أَبِى هُرَيْرَةَ قَالَ كَانَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- يُرَغِّبُ فِى قِيَامِ رَمَضَانَ مِنْ غَيْرِ أَنْ يَأْمُرَهُمْ فِيهِ بِعَزِيمَةٍ فَيَقُولُ « مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ ». فَتُوُفِّىَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- وَالأَمْرُ عَلَى ذَلِكَ ثُمَّ كَانَ الأَمْرُ عَلَى ذَلِكَ فِى خِلاَفَةِ أَبِى بَكْرٍ وَصَدْرًا مِنْ خِلاَفَةِ عُمَرَ عَلَى ذَلِكَ.

It was narrated that Abu Hurairah said: "The Messenger of Allah (s.a.w) used to encourage them to pray Qiyam in Ramadan without saying that it was obligatory. He would say: 'Whoever prays Qiyam in Ramadan out of faith and in the hope of reward, his previous sins will be forgiven.' Then the Messenger of Allah (s.a.w) died and that is how it was, and it remained like that throughout the Khilafah of Abu Bakr and at the beginning of the Khilafah of 'Umar."