He should perform two prostrations before the salutation, as it was mentioned by Sulaiman b. Bilal.
Sahih Muslim Book 5, Hadith 111
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The Messenger of Allah (ﷺ) said: When any one of you is in doubt about his prayer and he does Dot know how much he has prayed, three or four (rak'ahs). he should cast aside his doubt and base his prayer on what he is sure of. then perform two prostrations before giving salutations. If he has prayed five rak'ahs, they will make his prayer an even number for him, and if he has prayed exactly four, they will be humiliation for the devil.
Sahih Muslim Book 5, Hadith 110 Next Hadith → The Messenger of Allah (ﷺ) said the prayer; (the narrator added): He made some act of omission or commission when he pronounced salutation; it was said to him: Messenger of Allah, is there something new about the prayer? He (the Holy Prophet) said: What is it? They said: You said prayer in such and such away. He (the narrator) said: He (the Holy Prophet) turned his feet and faced the Qibla and performed two prostrations and then pronounced salutations, and then turned his face towards us and said: If there is anything new about prayer (new command from the Lord) I informed you of that. But I am a human being and I forget as you for. get, so when I forget, remind me, and when any one of you is in doubt about his prayer. he should aim at what Is correct. and complete his prayer in that respect and then make two prostrations.
Sahih Muslim Book 5, Hadith 112
Sahih Muslim Book 5, Hadith 110 Next Hadith → The Messenger of Allah (ﷺ) said the prayer; (the narrator added): He made some act of omission or commission when he pronounced salutation; it was said to him: Messenger of Allah, is there something new about the prayer? He (the Holy Prophet) said: What is it? They said: You said prayer in such and such away. He (the narrator) said: He (the Holy Prophet) turned his feet and faced the Qibla and performed two prostrations and then pronounced salutations, and then turned his face towards us and said: If there is anything new about prayer (new command from the Lord) I informed you of that. But I am a human being and I forget as you for. get, so when I forget, remind me, and when any one of you is in doubt about his prayer. he should aim at what Is correct. and complete his prayer in that respect and then make two prostrations.
Sahih Muslim Book 5, Hadith 112