He said, "Were it not for two things, I would have loved to be a slave. That is because I heard the Messenger of Allah ﷺ say, 'Allah does not create a servant who fulfills the right of Allah and the right of his master without Allah granting him his reward twice.'"Yazid said, "A slave cannot do anything with his property."
حدثنا يزيد بن هارون وهاشم قالا أخبرنا ابن أبي ذئب عن المقبري عن أبي هريرة قال هاشم في حديثه عن أبيه
قال لولا أمران لأحببت أن أكون مملوكا وذلك أني سمعت رسول الله ﷺ يقول ما خلق الله عبدا يؤدي حق الله وحق سيده إلا وفاه الله أجره مرتين قال يزيد إن المملوك لا يستطيع أن يصنع في ماله شيئا
Its isnad is sahih. Hashim is Ibn Qasim, mentioned previously. The hadith is found in Bukhari, 5/175, no. 2548, Fath, in the chapter on manumission, "when a slave performs his worship of his Lord well."