He who observed the fasts of Ramadan with faith and seeking reward (from Allah), all his previous sins would be forgiven, and he who observed prayer on Lailat-ul- Qadr with faith and seeking reward (from Allah), all his previous sins would be forgiven. (Using translation from Muslim 760a)  

أَنَّ النَّبِيَّ ﷺ قَالَ «مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ وَمَنْ قَامَ لَيْلَةَ الْقَدْرِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ»  

We fasted with the Messenger of Allah ﷺ during Ramadan, but he did not make us get up at night for prayer at any time during the month till seven nights remained; then he made us get up for prayer till a third of the night had passed. When the sixth remaining night came, he did not make us get up for prayer. When the fifth remaining night came, he made us stand in prayer till a half of the night had gone. So I said: Messenger of Allah, I wish you had led us in supererogatory prayers during the whole of tonight. He said: When a man prays with an imam till he goes he is reckoned as having spent a whole night in prayer. On the fourth remaining night he did not make us get up. When the third remaining night came, he gathered his family, his wives, and the people and prayed with us till we were afraid we should miss the falah (success). I said: What is falah? He said: The meal before daybreak. Then he did not make us get up for prayer during the remainder of the month. (Using translation from Abū Dāʾūd 1375)   

صُمْنَا مَعَ رَسُولِ اللَّهِ ﷺ شَهْرَ رَمَضَانَ قَالَ فَلَمْ يَقُمْ بِنَا مِنَ الشَّهْرِ شَيْئًا حَتَّى بَقِيَ سَبْعٌ قَالَ فَقَامَ بِنَا حَتَّى ذَهَبَ ثُلُثُ اللَّيْلِ قَالَ فَلَمَّا كَانَتِ السَّادِسَةُ لَمْ يَقُمْ بِنَا فَلَمَّا كَانَتِ الْخَامِسَةُ قَامَ بِنَا حَتَّى ذَهَبَ شَطْرُ اللَّيْلِ الْآخِرُ قُلْنَا يَا رَسُولَ اللَّهِ لَوْ نَفَّلْتَنَا بَقِيَّةَ هَذِهِ اللَّيْلَةِ فَقَالَ «إِنَّ الرَّجُلَ إِذَا قَامَ مَعَ الْإِمَامِ حَتَّى يَنْصَرِفَ مِنْ صَلَاتِهِ حُسِبَ لَهُ قِيَامُ لَيْلَتِهِ» فَلَمَّا كَانَتِ الرَّابِعَةُ لَمْ يَقُمْ بِنَا فَلَمَّا كَانَتِ الثَّالِثَةُ جَمَعَ أَهْلَهُ وَنِسَاءَهُ وَالنَّاسَ فَقَامَ بِنَا حَتَّى خَشِينَا أَنْ يَفُوتَنَا الْفَلَاحُ قُلْنَا وَمَا الْفَلَاحُ قَالَ السُّحُورُ قَالَ ثُمَّ لَمْ يَقُمْ بِنَا بَقِيَّةَ الشَّهْرِ  

[Machine] "From Abu Dharr, towards him."  

عَنْ أَبِي ذَرٍّ نَحْوَهُ