Musannaf Ibn Abi Shayba 22:198

Al-Thaqafi narrated to us, from Shu'ayb, who said: Abu al-'Aliyah used to teach people the Qur'an. When he wanted to correct [someone], he would not say, "It is not such and such," but rather he would say, "Recite such and such verse." I mentioned this to Ibrahim, and he said: "I think your companion has heard that whoever disbelieves in one letter of it has disbelieved in all of it."