This du’ah is not mandatory. It is Sunnah. Therefore, you prayer is correct.

Is the prayer valid if the person forgets to read Thanaa at the beginning?

Prayer | March 25, 2014 4:14 pm
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While I was praying sunnah prayer of Dhuhr prayer the Iqama for the fard saalah started. At that moment I was in the sujood of first rakah. After completing the sujood I left the saalah without taslim for joinning fard saalah. Is this ok?