Musannaf Ibn Abi Shayba 28:126

Ibn ‘Ulayyah narrated to us, from ‘Awf, who said: ‘Abbad narrated to me, from Bajalah ibn ‘Abdah, who said: ‘Umar wrote to Abu Musa: "Offer to those among you who are Magians that they abandon the marriage of their mothers, their daughters, and their sisters, and that they eat together, so that they may join the People of the Book. And kill every sorcerer and soothsayer."