Abd al-Rahman bin Yazid narrates: I saw Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) erasing the Mu'awwidhatayn (Surah al-Falaq and Surah al-Nas) (from his mushaf), and he was saying: "Why do you add to it what is not in it?"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9116