Is saying "A'udhu billahi minhu" (I seek refuge in Allah from it/him) always between the two prostrations considered an impermissible addition to the prayer?

The supplication between the two prostrations is mentioned in fatwa number: 32290. It should be memorized and consistently recited because it is one of the obligatory acts of prayer. As for the phrase: "I seek refuge in Allah from it," if the intention is to seek refuge in Allah from Satan, or from Allah's wrath and punishment, then there is no harm in that, and it is not considered an addition, because seeking refuge is among the prescribed remembrances in prayer. However, one should limit oneself to what has been authentically reported.