Answered by Shaykh Yusuf Weltch
Is it considered an oath, and is Kaffara (expiation) required if someone makes a dua promising not to commit a sin but does not mention Allah’s name?
In the Name of Allah, the Most Merciful and Compassionate
A promise is different than a vow or an oath. If one does not swear by the Name of Allah Most High, this does not obligate fulfillment such that expiation is due for non-fulfillment. [Maydani, al-Lubab fi Sharh al-Kitab]
A promise, however, should not be taken lightly, and one should try their best to fulfill their promises. This is even more important when the promise is made to Allah Most High.
Constant non-fulfillment of promises, breaking of promises intentionally, and neglecting the fulfillment of promises is sinful and a sign of hypocrisy. [Nahlawi, al-Durar al-Mubaha]
The Messenger of Allah (may Allah bless him and give him peace) said, “The signs of the hypocrite are three: when they speak, they lie; when they promise, they break their promise; and when they are entrusted, they betray that trust.” [Bukhari; Muslim]
Hope this helps
Allah knows best
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani
Shaykh Yusuf Weltch is a teacher of Arabic, Islamic law, and spirituality. After accepting Islam in 2008, he completed four years at the Darul Uloom seminary in New York, where he studied Arabic and the traditional sciences. He then traveled to Tarim, Yemen, where he studied for three years in Dar Al-Mustafa under some of the most outstanding scholars of our time, including Habib Umar Bin Hafiz, Habib Kadhim al-Saqqaf, and Shaykh Umar al-Khatib. In Tarim, Shaykh Yusuf completed the memorization of the Quran and studied beliefs, legal methodology, hadith methodology, Quranic exegesis, Islamic history, and several texts on spirituality. He joined the SeekersGuidance faculty in the summer of 2019.