What is the ruling on absentmindedness in prayer due to involuntary mental wandering?

The worshipper should cast aside worldly preoccupations and turn to the worship of his Lord with humility, contemplating what he recites from the Qur’an and supplications. If Satan whispers to him, he should seek refuge in Allah from him and spit dryly to his left three times. Uthman ibn Abi al-As did this when he complained to the Prophet, peace and blessings be upon him, about the whispers of the devil called "Khanzab" in his prayer and recitation, and Allah removed that from him.