Assalam o alaikum respected Mufti sahib!
I hope you are well.I have a question regarding salah.Today as I prayed two rakaah farz of Fajr,in the second rakah I feel(its a doubt,I am not sure) like I went directly into sajda after Qiraat,and that I did not do ruku and the posture after ruku in which; Sami alah hu liman hamida rabbana lakal hamd; is recited.So in this case, should i repeat the salah? Keep in mind its a doubt and not certainty.
In the name of Allāh, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-raḥmatullāhi wa-barakātuh.
In principle, if a person omits a Fardh act in Salaah intentionally or unintentionally, the Salaah will be void and the Salaah has to be repeated.
Ruku is from amongst the Faraidh of Salaah, since you perceived a doubt with regards to this posture, it is best that the Salaah be repeated.
And Allah Ta‘ala knows best
Mufti Luqman Hansrot
 والأركان من المذكورات أربعة القيام والقراءة والركوع والسجود وقيل القعود الأخير مقدار التشهد.
(نور الإيضاح ونجاة الأرواح,دار الحكمة, ج1 , ص42)
(Bahishti Zewar, Publ Altaf & Sons, Vol 1, Pg 156)