If a person does not recite “allahumma salli ala Muhammadiw wa ali Muhammad” in the tashahhud of his salaat. Is the salaat valid?

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I submitted this question to the office of Ayatollah Sayed Sistani and the reply i got was this:

If the person did not know the ruling and was not negligent in learning it, then he does not have to perform Qada’.

If a person does not recite “allahumma salli ala Muhammadiw wa ali Muhammad” in the tashahhud of his salaat. Is the salaat valid?

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If the person deliberately did not recite it then yes the Salat is invalid. But if someone forgot to recite it then it is valid.