Prayer of one who has an unpleasant smell in his mouth
Question

                If I pray and the smell in my mouth is unpleasant, is my prayer valid?.
Praise be to Allah.The one who is praying is 
    conversing with his Lord, as the Messenger of Allaah  (peace and 
    blessings of Allaah be upon him) said: “When one of you stands to pray, he 
    is conversing with his Lord” Narrated by al-Bukhaari (390) and Muslim (762). 
    So he should be fully prepared for standing before his Lord, and his body 
    and clothes and the place where he prays should be clean and free of any 
    offensive smells. Allaah says (interpretation of the meaning): 
“O Children of Adam! 
    Take your adornment (by wearing your clean clothes) while praying [and going 
    round (the Tawaaf of ) the Ka‘bah] and eat and drink but waste not by 
    extravagance, certainly He (Allaah) likes not Al‑Musrifoon (those who waste 
    by extravagance)”
[al-A’raaf 7:31]. 
So the worshipper is 
    enjoined to adorn himself and look presentable when praying. 
And the Prophet  (peace 
    and blessings of Allaah be upon him) said: Were it not for the fact that I 
    did not want to make things too hard for my ummah, I would have commanded 
    them to use the siwaak at every time of prayer.” Narrated by al-Bukhaari 
    (838)and Muslim (370). 
With regard to the validity 
    of prayer when there is an unpleasant smell in the mouth, it is valid but it 
    is makrooh (disliked). 
If the prayer is offered in 
    the mosque and this smell is obvious to such an extent that it annoys other 
    worshippers and the angels, then it is haraam. Al-Bukhaari (806) and Muslim 
    (870) narrated that the Prophet  (peace and blessings of Allaah be upon 
    him) said: “Whoever has eaten of this plant (i.e., garlic), let him not 
    approach our mosque.” 
And he  (peace and 
    blessings of Allaah be upon him) said: “Whoever has eaten of this plant let 
    him not approach our mosque and annoy us with the smell of garlic.” Narrated 
    by Maalik in al-Muwatta’ (27). 
Our advice is to pay 
    attention to cleaning your mouth, and if it needs some treatment then have 
    it treated so that you will not be embarrassed and keep away from prayer in 
    congregation.  
And Allaah knows best.