Is it necessary to repeat the Rak'ah or is it sufficient to perform Sujood As-Sahw when in doubt about the number of prostrations or Rak'ahs during prayer?

If a worshipper doubts whether he performed one prostration or two, and he has not yet begun the bowing of the next rak'ah, then he should assume the lesser number, return, and perform a second prostration. The same applies to doubt concerning bowing: he should return and bow, unless he has already begun the bowing of the second rak'ah, in which case the rak'ah containing the error is invalidated and the subsequent rak'ah takes its place, and he performs the prostrations of forgetfulness (sujood as-sahw). The evidence for this is the hadeeth of Abu Sa'eed: "If any of you doubts in his prayer and does not know how many rak'ahs he has prayed—three or four—let him cast aside doubt and build upon what he is certain of, then prostrate twice before giving the salutation."