Oaths are a magnification of the Creator: Imam al-Sa’di
In one section of his explanation of sheikh Muhammad ibn ‘Abd al-Wahhaab’s famous Kitaab al-Tawheed, sheikh ‘Abd al-Rahman ibn Naasir al-Sa’di wrote the following regarding oaths:
أصل اليمين إنما شرعت تأكيدا للأمر المحلوف عليه، وتعظيما للخالق، ولهذا وحب أن لا يحلف إلا لله، وكان الحلف بغيره من الشرك. ـ
The basis of oaths is that they were legislated as an affirmation of an issue which is being sworn to and as a magnification of the Creator. So for that reason, it is incumbent that one does not swear by anything but Allah, and swearing by other than Him is from shirk.
ومن تمام هذا التعظيم أن لا يحلف بالله إلا صادقا. ومن تمام هذا التعظيم أن يحترم اسمه العظيم عن كثرة الحلف ، فالكذب وكثرة الحلف تنافي التعظيم الذي هو روح التوحيد. ـ
And from the completion of this magnification is that one does not swear by Allah in any other way than truthfully. And from the completion of this magnification is that one venerates His magnificent name by not being prolific in swearing. For lying and a proliferation of swearing negates the magnification which is the essence of al-tawheed.
[al-Qawl al-Sadeed fi Maqaasid al-Tawheed pg. 182, corresponding to chapter 62]
See also: The Fiqh of Oaths – Imam al-Sa’di