משנה על בבא בתרא 9:2

One who assigns in writing his possessions to his slave, [the slave] thereby goes free. If he reserved for himself any land, however small, he does not become free. Rabbi Shimon says: he always becomes free, unless [the master] says: “Behold, all my goods are given to so-and-so my slave, with the exception of one ten-thousandth part of them.”