Does the prayer become invalid if a piece of the lip is bitten off by the teeth and unconsciously kept in the mouth during prayer?

Intentionally putting something in the mouth during prayer is disliked because it compromises solemnity (khushu'), like sticking out the tongue or opening the mouth. However, intentionally swallowing or chewing something invalidates the prayer. Similarly, if one puts something in their mouth that dissolves and mixes with saliva, it invalidates the prayer. For example, if one chews gum, their prayer is invalidated. If they do not chew it but merely put it in their mouth, and it is new and dissolves, their prayer is invalidated. If it is used and does not dissolve, it does not invalidate the prayer.

As for instances of forgetfulness or inadvertence, there is no consequence for the worshipper, whether they swallowed a piece of hide or chewed it, as long as it was a small amount. If it was a large amount, the prayer is invalidated, according to the more sound of the two opinions. The distinction between small and large amounts is determined by common practice.

If one keeps it in their mouth after remembering, without chewing, the prayer is not invalidated by that, even if done intentionally, as long as nothing from it dissolves and enters the stomach with the saliva. Although doing so is disliked and compromises solemnity, it is preferable to remove it after remembering.