كتاب فضائل الصحابة رضى الله تعالى عنهم
44
The Book of the Merits of the Companions
(52)
Chapter: The Virtues Of the Sahabah, Then Those Who Come After Them, Then Those Who Come After Them
(52)
باب فَضْلِ الصَّحَابَةِ ثُمَّ الَّذِينَ يَلُونَهُمْ ثُمَّ الَّذِينَ يَلُونَهُمْ ‏‏

This hadith has been narrated on the authority of 'Imran b. Husain through another chain of transmitters (and the words are):

The best generation of this Umma is the generation to which I have been sent, then the next one, and there is an addition in the hadith transmitted on the authority of Abu 'Awana (and the words are): And Allah knows best whether he made a mention of the third (generation) or not; the rest of the hadith is the same as transmitted by Zahdam on the authority of 'Imran. And in the hadith transmitted by Hisham on the authority of Qatada there is an addition of these words: They take an oath whereas they are not asked to take.
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، وَمُحَمَّدُ بْنُ عَبْدِ الْمَلِكِ الأُمَوِيُّ، قَالاَ حَدَّثَنَا أَبُو عَوَانَةَ، ح وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُعَاذُ بْنُ هِشَامٍ، حَدَّثَنَا أَبِي كِلاَهُمَا، عَنْ قَتَادَةَ، عَنْ زُرَارَةَ بْنِ أَوْفَى، عَنْ عِمْرَانَ بْنِ حُصَيْنٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم بِهَذَا الْحَدِيثِ ‏"‏ خَيْرُ هَذِهِ الأُمَّةِ الْقَرْنُ الَّذِينَ بُعِثْتُ فِيهِمْ ثُمَّ الَّذِينَ يَلُونَهُمْ ‏"‏ ‏.‏ زَادَ فِي حَدِيثِ أَبِي عَوَانَةَ قَالَ وَاللَّهُ أَعْلَمُ أَذَكَرَ الثَّالِثَ أَمْ لاَ ‏.‏ بِمِثْلِ حَدِيثِ زَهْدَمٍ عَنْ عِمْرَانَ وَزَادَ فِي حَدِيثِ هِشَامٍ عَنْ قَتَادَةَ ‏"‏ وَيَحْلِفُونَ وَلاَ يُسْتَحْلَفُونَ ‏"‏ ‏.‏
Reference : Sahih Muslim 2535 c
In-book reference : Book 44, Hadith 306
USC-MSA web (English) reference : Book 31, Hadith 6158
  (deprecated numbering scheme)