Is the prostration of forgetfulness (sujud al-sahw) obligatory if the worshipper does not know how many rak'ahs he has prayed, and does it include the tashahhud? And is the prayer valid if the worshipper recites a surah with the Fatihah in the last two rak'ahs of a four-rak'ah prayer?

One who doubts in his prayer and does not know how many rak'ahs he has performed, should seek refuge with Allah from Satan and spit (lightly blow) to his left three times. Then, he should build upon the minimum (what he is certain of), complete his prayer, and perform two prostrations of forgetfulness (sujud as-sahw) before the salutation (taslim). As for reciting a surah with Al-Fatihah in the last two rak'ahs of a four-rak'ah prayer and the third rak'ah of Maghrib prayer, it is preferable to omit it and suffice with Al-Fatihah only.