If an Imam, while leading Salāh, lifts up both hands to straighten his collar/hat, would his and the Muqtadī’s Salāh be valid?
I heard that if both hands are lifted and it looks to an outsider like he is not praying the Salāh is not done. Please confirm as this has happened in a Masjid during Jamāh.
As-salāmu ‘Alaykum Wa-rahmatullāhi Wa-barakātuh.
In the Name of Allāh, the Most Gracious, the Most Merciful.
In principle, salāh is invalidated if a person does ‘Aml Kathīr (excessive movement) during Salāh. If the person does ‘Aml Qalīl (minor movement) then the salāh will remain valid.
Although using both hands to do an action is one of the definitions of ‘Aml Kathīr in Hanafī Madhab, yet the most authentic view is that it is an act which makes an onlooker confident that the person doing the act is not in salāh. If the onlooker only has a doubt, then the act will be ‘Aml Qalīl.
In the case you have mentioned, the act of raising both hands to fix a collar or hat is such that an onlooker will not consider the Imām to be in state of salāh. Therefore, the imām’s salāh will be invalidated by this act.
The salāh of muqtadīs (followers) depend on the salāh of the Imām. Hence the whole congregation will have to repeat the salāh. 
We advise you to approach the Imām and inform him of the issue in a very polite and respectable manner, and then request him to inform the congregation which was present at that particular salāh in the best manner possible.
And Allah Ta‘ala Knows Best
Mufti Faisal bin Abdul Hameed,
Checked and Concurred by,
Mufti Luqman Hansrot