Abdullah bin al-Mukaddam al-Thaqafi (may Allah have mercy on him) says that when the Messenger of Allah (peace and blessings be upon him) besieged the people of al-Ta'if, some of their slaves came out to him: Sayyiduna Abu Bakrah (may Allah be pleased with him), who was the slave of al-Harith bin Kaladah, and al-Munba'ith, Yuhannas and Wardan, in a group of their slaves, and they accepted Islam.
When the delegation of the people of al-Ta'if came to the Messenger of Allah (peace and blessings be upon him) and accepted Islam, they said: "O Messenger of Allah (peace and blessings be upon him)! Return to us our slaves who came to you." He (peace and blessings be upon him) said: "They will not be returned, for they are the freed ones of Allah." He only gave them the attribution of the wala' (clientship) of their slaves.
This report is munqati' (its chain is broken).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18839