Sayyiduna Abu Umamah (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) said: "I advise the caliphs who come after me to fear Allah, and I advise him concerning the community of the Muslims: that he honour their elders and show compassion to their young, that they honour their scholars, that they neither harm them nor humiliate them, that they do not frighten them so that they turn to disbelief, that they do not castrate them so that their lineage is cut off, and that they do not shut their doors (to them) so that the strong devour the weak."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16644