What is the ruling on spitting to the left after always beginning the prayer (Istiftah), and does this invalidate previous prayers, given the presence of scrupulosity (waswasa)?

It is legislated for the worshipper, if Satan whispers to him, to seek refuge in Allah from him and to lightly spit to his left three times, inclining his head to the left, and to say in a low voice: "A'udhu billahi minash-shaytanir-rajim (I seek refuge in Allah from the accursed Satan)."

If the worshipper is part of a congregation, he may spit to his left into his garment, or ghutra (head covering), or a handkerchief. If this is not possible, he should suffice with turning his head to the left and saying: "A'udhu billahi minash-shaytanir-rajim."

Spitting during prayer without need or whispers is disliked (makrooh), and it invalidates the prayer if done intentionally and with sound.