Sahih Al Bukhari – Book of Good Manners And Form (Al-Adab)

Volumn 008, Book 073, Hadith Number 002. 
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Narated By Abu Huraira RA: A man came to Allah's Apostle PBUH and said, "O Allah's Apostle! Who is more entitled to be treated with the best companionship by me?" The Prophet said, "Your mother." The man said. "Who is next?" The Prophet said, "Your mother." The man further said, "Who is next?" The Prophet said, "Your mother." The man asked for the fourth time, "Who is next?" The Prophet said, "Your father."