if I join imam in silent prayer and I dont know which rakah is this one,should i soppose to ricite TAUZ ,TASMIYYA AND SANA ? even if it is third rakah?
thank alot

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

It is permissible for a person who is preceded in a silent prayer [Thuhr or ‘Asr] whether or not he knows in which Rak’ah the Imaam is, to say the Isti’aathah [seeking refuge in Allaah from the devil] and the Basmalah [which is to say: Bismillaahi Ar-Rahmaani Ar-Raheem (In the Name of Allaah, The Most Gracious, The Most Merciful)] because he is permitted to recite the Quran in the prayer. Sufyaan Ath-Thawri was asked: “Should a person led in prayer seek refuge in Allaah from the devil”, and he replied: “The one who recites the Quran should seek refuge in Allaah from the devil.” Imaam Ahmad said: “Indeed, he (Sufyaan) spoke the truth.”

On the other hand, it is permissible for such a person to say the Istiftaah (the opening supplication for the prayer) if he joins the Imaam in the first Rak’ah or any other Rak’ah, and he predominantly thinks that mentioning this supplication would not make him miss reciting Al-Faatihah [the opening Chapter of the Quran] before the Imaam that leads the prayer bows down for Ruku’ (bowing down position), however, if he fears that if he recites this supplication he would miss the recitation of Al-Faatihah, then he should start reciting it and not mention the opening supplication for the prayer, because the recitation of Al-Faatihah is an obligation (Waajib) whereas reciting the opening supplication for the prayer is a Sunnah.

However, if a person who is preceded in prayer joins the Imaam in a position other than the standing position, whether it is in Ruku’ or Sujood [prostration] or Tashahhud position, then he should say the Takbeeratul Ihraam [saying Allaahu Akbar to start the prayer] and then he should recite the mention that the Imaam is saying [i.e. he joins the Imaam in whichever position he is and mentions the appropriate mention of Allaah that the Imaam is supposed to say] but he should not say the opening supplication of the prayer neither straight away nor afterwards.

Finally, if a person joins the Imaam in the standing position, he should say the Istiftaah, Isti’aathah and Basmalah if he thinks he has time to do so even if the Imaam is in the last Rak’ah because whatever a person who is preceded in the prayer reaches with the Imaam, is considered as the beginning of his prayer [prayer of a person who is preceded in the prayer.]

Allaah Knows best.