Depends on what extend.
However you will not be rewarded for that prayer except the part you were concentrating in.

Sometimes when I pray I intentionally think about other things, does this make the salah invalid?

Prayer | November 15, 2015 9:36 pm
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Shaik ..Is it okay to say in ruku and sajdah, after subhana rabbiyal ala..(subhanaka Allahumma rabbana Wa behamdik Allahumma figrilli.) Please Advice..
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I joined imam in 2nd rakah in magrib salah. Imam offered sajda sahu & I also offered & I completed my salah by praying one rakah after lmam said the last tasleem. Later on people told me that imam had not recited fatihah in 1st rakah but only surah. Is my and other people’s prayer valid or we have to repeat it?