Ruling on the fasting of one who only prays in Ramadaan
Question

                What is the ruling on the fasting of one who only prays in Ramadaan, or who maybe fasts and does not pray?.
Praise be to Allah.
    If a person is deemed to be a kaafir, all his deeds are 
    invalidated. Allaah says (interpretation of the meaning): 

    “But if they had joined in worship others with Allaah, all 
    that they used to do would have been of no benefit to them”

    [al-An’aam 6:88] 

    “And whosoever 
    disbelieves in Faith, [i.e. in the Oneness of Allaah and in all the other 
    Articles of Faith i.e. His (Allaah’s) Angels, His Holy Books, His 
    Messengers, the Day of Resurrection and Al‑Qadar (Divine Preordainments)], 
    then fruitless is his work; and in the Hereafter he will be among the losers”

    [al-Maa’idah 5:5]. 

    A number of scholars are of the view that he is a not a 
    kaafir in the sense of major kufr if he affirms that prayer is obligatory, 
    but he is guilty of minor kufr, and this action of his is worse and more 
    abhorrent than the action of the adulterer or thief and the like. Despite 
    that, his fasting and Hajj are still valid, in their view, if he does them 
    in the proper manner as prescribed in Islam. But his offence is that he is 
    not praying regularly, and he is in grave danger of falling into major shirk 
    according to a number of scholars. Some of them narrated that the majority 
    are of the view that he is not a kaafir in the sense of major kufr if he 
    does not pray because he is lazy or heedless, rather he is committing minor 
    kufr thereby, but his crime is still serious and an abhorrent evil that is 
    worse than adultery, stealing, disobeying one’s parents, drinking alcohol 
    and so on. We ask Allaah to keep us safe and sound. 

    But the more correct of the two scholarly views is that he is 
    a kaafir in the sense of major kufr – we ask Allaah to keep us safe and 
    sound – because of the shar’i evidence quoted above. So if a person fasts 
    but does not pray, his fasting does not count, and neither does his Hajj. 
    End quote.