Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) says that on the day of al-Hudaybiyyah, before the treaty, two slaves came to the Messenger of Allah (peace and blessings be upon him). Their masters wrote: O Muhammad! They have not come out of desire for your religion; they only seek release from slavery. The people said: O Messenger of Allah (peace and blessings be upon him)! They have spoken the truth; let the slaves be returned to them. The Messenger of Allah (peace and blessings be upon him) became angry and said: "O people of Quraysh! I do not think you will desist until Allah, the Lord of Might, sets over you someone who will strike your necks." So he (peace and blessings be upon him) did not return those slaves and said: "They are the freed ones of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18838