It was said to ‘Ali (رضي الله عنه): “Did your Messenger say anything to you only that was not for the people in general?” He said: “The Messenger of Allah (ﷺ) did not say anything to us only that he did not say to the people, except something in the sheath of this sword of mine. He took out a document on which there was something about the ages of camels [for zakah] and in it was said: "Madinah is sacred, the area between Thawr and ‘A’ir. Whoever commits an offence or gives refuge to an offender, upon him will be the curse of Allah, the angels and all the people, and on the Day of Resurrection Allah will not accept any nafl or obligatory act of worship from him. Protection given by any Muslim is binding upon all of them. Whoever transgresses protection given by a Muslim, upon him be the curse of Allah, the angels and all the people, and on the Day of Resurrection Allah will not accept any nafl or obligatory act of worship from him. Whoever takes people as mawla without the permission of the ones who set him free, upon him be the curse of Allah, the angels and all the people, and on the Day of Resurrection Allah will not accept any nafl or obligatory act of worship from him."

The word mawla refers to the one who manumits a slave, who has a right to inherit ex-slave. Changing one’s mawla means giving the right of inheritance to the new mawla.