Aisha reported: The Prophet, peace and blessings be upon him, said, “Gabriel continued to advise me to treat neighbors well until I thought he would make them my heirs.”

Source: Sahih Bukhari 5668, Sahih Muslim 2624

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

عَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ مَا زَالَ يُوصِينِي جِبْرِيلُ بِالْجَارِ حَتَّى ظَنَنْتُ أَنَّهُ سَيُوَرِّثُهُ

5668 صحيح البخاري كتاب الأدب باب الوصاة بالجار

2624 صحيح مسلم كِتَاب الْبِرِّ وَالصِّلَةِ وَالْآدَابِ بَاب الْوَصِيَّةِ بِالْجَارِ وَالْإِحْسَانِ إِلَيْهِ

http://dailyhadith.abuaminaelias.com/2011/06/30/gabriel-advises-prophet-muhammad-to-treat-neighbors-well/