Putting gum in the mouth without chewing it when praying
Question

                Is it permissible to leave gum in the mouth during prayer without chewing it?.
Praise be to Allah.A person should focus fully 
    on his prayer and have proper presence of mind, and remember that he is 
    conversing with his Lord, as al-Bukhaari (405) and Muslim (551) narrated 
    from Anas ibn Maalik that the Prophet  (peace and blessings of Allaah be 
    upon him) saw some sputum in the qiblah (of the mosque) and that upset him 
    so much that it could be seen on his face. He scratched it with his hand 
    then he said: “When one of you stands to pray, he is conversing with his 
    Lord, or his Lord is between him and the qiblah, so you should not spit in 
    the direction of the qiblah, rather to your left or beneath your feet.” Then 
    he took the edge of his garment and spat in it, then rubbed part of it 
    against another part, and he said: “Or do like this.”  
Leaving gum in the mouth at 
    the very least is contrary to the etiquette of standing before Allaah, may 
    He be exalted, and it may prevent one from reciting properly, or distract 
    the worshipper. If he chews it or if some of it dissolves, then his prayer 
    is invalidated.   
Al-Nawawi (may Allaah have 
    mercy on him) said in al-Majmoo’ (4/23): al-Baghawi and others said: 
    Chewing alone invalidates the prayer, even if nothing reaches the stomach, 
    and if a person chews mastic, his prayer is invalidated. If he does not chew 
    it, rather he puts it in his mouth, then if it is new and dissolves it is 
    like sugar and his prayer is invalidated according to the correct view, but 
    if it is used and it does not dissolve, then it does not invalidate his 
    prayer, just as if he held a pebble or a pear in his mouth, it dos not 
    invalidate his prayer. End quote. 
And Allaah knows best.