It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the words by which the Prophet (peace and blessings be upon him) most often swore were these: «لا و مقلب القلوب» No, by the Turner of the hearts.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5347
Hadith Gradingحکم دارالسلام:إسناده صحيح ، خ : 6617 .