Bismihi Subhanahu Wa-ta'ala
Assalamooalaikum Warahmatullahi Wabarakatuh

Salaat should be read with Khushu' and Khudhu' (concentration and devotion) consequently, one should not unnecessarily fiddle with clothing as this would detract from the blessings of the prayers. One should make sure the trousers are above the ankles before the Salaah and if they were to slip down then one would not be responsible for this. If it does habitually slip down then one must track the source of this problem and ensure this does not happen. For example, if the trousers are loose around the waist then solve this problem by using a belt or adjusting the length of the trouser of tahband (lunghi) etc. In this case, one would not be plagued with such problems.

Despite taking the necessary precautions before Salaat, if one's trousers slip below the ankles then one should lift them up with one hand.

Wallahu A'alam