Is the opening supplication (Du'a al-Istiftah) in prayer obligatory or a good innovation (Bid'ah Hasana)?

The opening supplication (Du'a al-Istiftah) is not an innovation; rather, it is among the Sunnah acts of prayer for which one is rewarded, and prayer is not invalidated by omitting it. It has been authentically reported from the Prophet, peace and blessings be upon him, that he used to say: "O Allah, distance me from my sins as You have distanced the East from the West. O Allah, purify me from my sins..." and so on.