Is saying "Ya Subhanallah" (O Glory be to Allah) when astonished, instead of "Subhanallah" (Glory be to Allah), considered disbelief (kufr) or apostasy (riddah)? And does it necessitate re-observing the expiatory fast merely due to doubt about uttering it?

Saying "Ya Subhan Allah" (O Glory be to Allah) out of astonishment is permissible. The practice of the righteous predecessors (Salaf) supports it, and it has been reported in Marfu' hadiths from the Prophet, peace and blessings be upon him, as well as authentically narrated from companions such as Aisha and Umar, may Allah be pleased with them both. Therefore, this saying does not affect fasting or creed. The questioner is advised to avoid obsessive thoughts.