The majority of the scholars state that reciting the Isti`adhah (in the prayer and when reciting the Qur'an) is recommended and not required, and therefore, not reciting it does not constitute a sin. However, Ar-Razi recorded that `Ata' bin Abi Rabah said that the Isti`adhah is required in the prayer and when one reads the Qur'an. In support of `Ata's statement, Ar-Razi relied upon the apparent meaning of the Ayah,

﴿فَاسْتَعِذْ﴾

(Then seek refuge.) He said that the Ayah contains a command that requires implementation. Also, the Prophet always said the Isti`adhah. In addition, the Isti`adhah wards off the evil of Satan, which is neccessary, the rule is that the means needed to implement a requirement of the religion is itself also required. And when one says, "I seek refuge with Allah from the cursed devil.'' Then this will suffice.