Question: Is prayer valid behind someone who does not observe stillness in salaah, especially in bowing and sitting between the two prostrations. According to Hanafiyyah prayer in such instance is still valid but makruh tahrimi but would require repeating if done intentionally and sajdah sahw if out forgetfulness. Another view states required to repeat in all cases. Repeating would be done for compensation of sin.
Answer: Wa alaikum assalam wa rahmatullah,
I pray that you are well, insha’Allah.
Remaining motionless when standing from the bowing position (qawma) and between the two prostrations (jalsa) is necessary (wajib) according to the relied upon position in the Hanafi school. [Ibn Nujaym, al-Bahr al-Ra’iq; Shurunbulali, Maraqi al-Falah; Ibn `Abidin, Radd al-Muhtar]
But, you only have to remain motionless for a moment (the extent of saying SubhanAllah). Thus, if a person does this, the prayer is sound without dislike.
You should assume the validity of his prayer, and not investigate. Focus on your own prayer.
And Allah alone gives success.