Imam al-Shafi'i (may Allah have mercy on him) says that the Messenger of Allah (peace and blessings be upon him) enslaved Banu al-Mustaliq and Hawazin, who were Arab tribes, and afterwards freed them as a favour. The people of knowledge differ about this. Some claim that when the Prophet (peace and blessings be upon him) released the captives of Hawazin, he said: "If any Arabs were to be complete captives, it would have been these people of Hawazin; but it was to be captivity followed by ransom." Imam al-Shafi'i (may Allah have mercy on him) says: According to those who consider this hadith established, an Arab cannot be enslaved.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18067