In the Name of Allah, the Inspirer of Truth.
Assalamu Alaikum Warahmatullahi Wabarakatuh

There are 2 issues here.

Firstly the trousers must be above the ankles all the time (except when one is lying down or sitting down). The sin of having one's clothing below the ankes patently forbidden (for men) in, and out of Salaah.

Secondly, folding the trousers in Salaah, would almost certainly amount to excessive movement, and the Salaah would be rendered invalid. One must not do anything in Salaah which would affect one's conentration and devotion, which this most certainly would.

Narrated Abu Dharr: The Messenger of Allah observed: "Three are the (persons) with whom Allah would neither speak on the Day of Resurrection, nor would look at them nor would absolve them, and there is a painful chastisement for them." The Messenger of Allah repeated it three times. Abu Dharr remarked: "They failed and they lost; who are these persons, Messenger of Allah?" Upon this he observed: "They are: the one who makes (his garment) hang down on the ground (al-musbil), the recounter of obligation, and the seller of goods by false oath."

Hence our advice to you is to keep your trousers above your ankle out of Salaah.

And Allah knows Best
Wa Alaykumussalaam Wa Rahmatullahi Wa Barakatuh
www.foodguide.org.uk