Hadith 510280
Chapter 22: Vows and Oaths - كتاب النذور والأيمان
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَقُولُ " لاَ وَمُقَلِّبِ الْقُلُوبِ " .
Yahya related to me from Malik that he had heard that the Messenger of Allah, may Allah bless him and grant him peace, used tosay, "No, by the Overturner of hearts."
Reference Muwatta Malik 510280
USC-MSA web (English) reference : Book 22, Hadith 15