Shaykh Muhammad ibn Saalih al-Uthaymeen rahimahullaah was asked the following question:
Q: What is the ruling on a person folding his sleeves during the prayer?
A: If he were to fold them because of the prayer then it enters into the saying of the Prophet sallAllaahu `alaihi wa sallam: “I have been commanded to prostrate on seven bones, and not to fold back clothing nor hair.”
[Agreed Upon; from a narration of Ibn Abbaas: Reported by al-Bukhaaree no. 810 & Muslim no. 490]
And as for if he had folded (his sleeves) before, due to some work – before he had entered into the prayer, or he folded them because of plentiful perspiration and the like; then that is not disliked.
As for if he folded them because they were long, then he should shorten them so that he does not fall into pride.
[Source: Majmoo` Fataawaa of Ibn Uthaymeen vol. 13 pge. 308]