What is the ruling on someone who is heedless in prayer and does not know whether he performed two prostrations or one prostration after rising from prostration, and what is then required of him?

If a worshiper doubts and does not know how many prostrations he has made, he should consider himself to have prostrated only once, then perform the second prostration, and then perform the prostrations of forgetfulness (sujud al-sahw) before the salutation (taslim); this is based on the Prophet Muhammad’s (peace and blessings be upon him) saying: "If any of you doubts in his prayer and does not know whether he has prayed three or four (rak'ahs), he should cast away doubt and build upon what he is certain of, then prostrate twice before giving the salutation."