Is It Obligatory to Fulfill an Oath Taken on a Mustahabb Act?

Source​

Taken from: Fatāwā Amunpuri by Shaykh Ghulām Muṣṭafā Ẓahīr Amunpuri

Question​

If one takes an oath regarding a recommended (mustaḥabb) act, is it necessary to fulfill that oath?

Answer​

If an oath has been taken regarding a permissible and recommended (mustaḥabb) act, then fulfilling it is obligatory. Otherwise, expiation (kaffārah) will become necessary.

Evidence from the Qur’an​

❀ Allah Almighty says:

﴿وَاحْفَظُوا أَيْمَانَكُمْ كَذَلِكَ يُبَيِّنُ اللَّهُ لَكُمْ آيَاتِهِ لَعَلَّكُمْ تَشْكُرُونَ﴾

Reference: al-Mā’idah: 89

Translation:
“Safeguard your (lawful) oaths. Thus Allah makes clear to you His verses so that you may give thanks.”