It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) was asked about these two surahs (al-Mu'awwidhatayn), and he said: "It was said to me, so I said it; so say as I said."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10174