Harming others does not invalidate a person's prayer but might cause the reward to be lost
9-2-2016 | IslamWeb
Question:
Assalaamu alaukum. I was arguing with my friends in the room. They wanted to sleep, and I wanted to study. So I switched on the light at my side and switched it off at the other side. They insisted on me switching off all of them. Then, during the argument, he told me that there is a hadith that states that a Muslim who disturbs other Muslims will have his prayers cancelled. Is there any hadith like that ? To be frank, I did not believe in that hadith because I do not think that he knows much about Islam. So I was narrating the incidence to a different guy. And then when I mentioned the hadith that he told me, we laughed. Does this mean that we mocked Islam? And does that makes us kafirs (disbelievers)? I laughed at it because I do not believe that there is any hadith like that. I think that he just wanted to use it to get me to turn off the lights. Also, the other day, a teacher told a Muslim and a Christian who were jokingly quarrelling in class about religious matters not to start their holy war in class. We laughed at what the teacher said, and anytime I remember what the teacher said, I laugh. Then, later on, I thought of the fact that a person who mocks Islam is a kafir and became really worried and disturbed. Please, did what the teacher say constitute kufr (disbelief)? And what is the ruling on laughing at what he said ?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad 
is His slave and Messenger.
We do not know of any hadeeth indicating that one's prayer is cancelled (invalidated) because he harms others. However, it has been reported that whoever harms others will be thrown into Hellfire even if he observes prayers and fasting. Abu Hurayrah 
narrated that the Prophet was asked, "O Messenger of Allaah! A certain woman prays at night and fasts in the day but she harms her neighbors with her tongue." He Messenger of Allaah said, "There is no good in her; she is one of the people of the Hellfire." [Ahmad and Al-Haakim]
However, harming people does not entail deeming prayers invalid; i.e. it does not mean that this woman was not rewarded for her deeds. Rather, perhaps she has earned the reward but will not benefit of it because her reward will be given to those whom she harmed with her tongue. It has been reported that the Prophet said, "Do you know who is the bankrupt?" They (the Companions) said, "The bankrupt among us is one who has neither money nor belongings." He said, "Verily, the bankrupt of my Ummah (community) will be he who will come on the Day of Resurrection with prayers and fasts and zakah, but he had hurled abuses upon some people, defamed some, unlawfully consumed the wealth of some, shed the blood of some, and beat some, so his rewards will be credited to the account of each of them (who suffered at his hand). And if his good deeds fall short to clear the account, then their sins will be entered in his account and he will be thrown in the Hellfire." [Muslim and Ahmad]
As for your laughter at what the teacher or your friend said, it does not constitute kufr because you were not laughing at what you believed to be part of the Quran or the Sunnah. Also, this teacher, if he is Muslim, is not judged to be a disbeliever by merely saying what he said because he, based on what is apparent, did not intend to ridicule the obligation of jihaad. Needless to say, takfeer (declaring a Muslim individual or group to be non-Muslim) is a serious and grave matter. It is impermissible for the Muslim to hasten to label his fellow Muslims disbelievers merely based on such remarks. For more benefit, please refer to fatwa 721.
Allaah knows best.