Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
suyuti:512a

Two (things) are found among men which are tantamount to unbelief: slandering one's lineage and lamentation on the dead. (Using translation from Muslim 67)   

السيوطي:٥١٢a

"اثنان في الناسِ هما بهم كُفْرٌ: الطعن في الأنساب، والنياحة على الميت".  

[حم] أحمد [م] مسلم عن أبى هريرة

See similar narrations below:

Collected by Muslim, Aḥmad
muslim:67[Chain 1] Abū Bakr b. Abū Shaybah > Abū Muʿāwiyah [Chain 2] Ibn Numayr > Abū And Muḥammad b. ʿUbayd > al-Aʿmash > Abū Ṣāliḥ > Abū Hurayrah

Two (things) are found among men which are tantamount to unbelief: slandering one's lineage and lamentation on the dead.  

مسلم:٦٧وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا أَبُو مُعَاوِيَةَ ح وَحَدَّثَنَا ابْنُ نُمَيْرٍ وَاللَّفْظُ لَهُ حَدَّثَنَا أَبِي وَمُحَمَّدُ بْنُ عُبَيْدٍ كُلُّهُمْ عَنِ الأَعْمَشِ عَنْ أَبِي صَالِحٍ عَنْ أَبِي هُرَيْرَةَ قَالَ

قَالَ رَسُولُ اللَّهِ ﷺ اثْنَتَانِ فِي النَّاسِ هُمَا بِهِمْ كُفْرٌ الطَّعْنُ فِي النَّسَبِ وَالنِّيَاحَةُ عَلَى الْمَيِّتِ  

ahmad:9690Muḥammad b. ʿUbayd > al-Aʿmash > Abū Ṣāliḥ > Abū Hurayrah

[Machine] The Messenger of Allah ﷺ said, "There are two things among people that are considered as disbelief: mourning loudly for the deceased and insulting lineage."  

أحمد:٩٦٩٠حَدَّثَنَا مُحَمَّدُ بْنُ عُبَيْدٍ قَالَ حَدَّثَنَا الْأَعْمَشُ عَنْ أَبِي صَالِحٍ عَنْ أَبِي هُرَيْرَةَ قَالَ

قَالَ رَسُولُ اللهِ ﷺ ثِنْتَانِ هُمَا بِالنَّاسِ كُفْرٌ نِيَاحَةٌ عَلَى الْمَيِّتِ وَطَعْنٌ فِي النَّسَبِ  

ahmad:10434Ibn Numayr > al-Aʿmash > Abū Ṣāliḥ > Abū Hurayrah

Two (things) are found among men which are tantamount to unbelief: slandering one's lineage and lamentation on the dead. (Using translation from Muslim 67)  

أحمد:١٠٤٣٤حَدَّثَنَا ابْنُ نُمَيْرٍ أَخْبَرَنَا الْأَعْمَشُ عَنْ أَبِي صَالِحٍ عَنْ أَبِي هُرَيْرَةَ قَالَ

قَالَ رَسُولُ اللهِ ﷺ اثْنَتَانِ فِي النَّاسِ هُمَا بِهِمْ كُفْرٌ الطَّعْنُ فِي النَّسَبِ وَالنِّيَاحَةُ عَلَى الْمَيِّتِ