Sahih Al Bukhari – Book of Wedlock, Marriage (Nikaah)

Volumn 007, Book 062, Hadith Number 074.
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Narated By Abu Huraira : The Prophet said, "Beware of suspicion (about others), as suspicion is the falsest talk, and do not spy upon each other, and do not listen to the evil talk of the people about others' affairs, and do not have enmity with one another, but be brothers. And none should ask for the hand of a girl who is already engaged to his (Muslim) brother, but one should wait till the first suitor marries her or leaves her."