Musannaf Ibn Abi Shaybah

Hasan says that if a person says to his slave, "Serve me for one year and you will be free," then the slave will serve for one year and then he will be free. And if he says to the slave, "You are free on the condition that you serve me for one year," then the owner dies, and after his death the slave serves his children for one year, then he is freed from one third of the owner's wealth.