A`ūdhu billāhi minash-shayṭānir-rajīm.
Seeking refuge with Allah against him (i.e. by saying I seek refuge in Allah from Satan the outcast).
Reference:
Abu Dawud 1/206, At-Tirmidhi. See also Al-Albani, Sahih At-Tirmidhi 1/77,
and Surat Al-Mu'minun, 23:98-9.