It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Jibril (peace be upon him) kept urging me so persistently about slaves (their rights and good treatment) that I began to think he would fix for them some definite limit or term such that, when they reached it (that stage or age), they would become free (by themselves)."
Musnad Ahmad: 2833. Allamah Shu'ayb al-Arna'ut graded it sahih li-ghayrihi (authentic due to supporting narrations) on the basis of supporting reports, because one narrator in its chain is unnamed.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب الأدب / 2833
Hadith Gradingمحدثین:إسناده صحيح
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :صحيح لغيره. » «انفرد به المصنف من هذا الطريق»