Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “Whoever follows the funeral of Muslim out of faith and seeking reward and he remains with it until the prayer is offered and the burial ceremony is finished, then he will return with two great rewards. Each reward is like the size of mount Uhud. Whoever offers the funeral prayer and returns before the burial, then he will return with one great reward.”

Source: Sahih Bukhari 47, Sahih Muslim 945

Grade: Muttafaqun Alayhi (authenticity agreed upon) according to Al-Bukhari and Muslim

عَنْ أَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ مَنْ اتَّبَعَ جَنَازَةَ مُسْلِمٍ إِيمَانًا وَاحْتِسَابًا وَكَانَ مَعَهُ حَتَّى يُصَلَّى عَلَيْهَا وَيَفْرُغَ مِنْ دَفْنِهَا فَإِنَّه يَرْجِعُ مِنْ الْأَجْرِ بِقِيرَاطَيْنِ كُلُّ قِيرَاطٍ مِثْلُ أُحُدٍ وَمَنْ صَلَّى عَلَيْهَا ثُمَّ رَجَعَ قَبْلَ أَنْ تُدْفَنَ فَإِنَّهُ يَرْجِعُ بِقِيرَاطٍ

47 صحيح البخاري كتاب الإيمان باب اتباع الجنائز من الإيمان

945 صحيح مسلم كتاب الجنائز باب فضل الصلاة على الجنازة واتباعها

http://dailyhadith.abuaminaelias.com/2014/03/28/hadith-on-funerals-great-rewards-for-those-who-follow-a-funeral-and-offer-prayers/