What is the ruling on one who refuses to apply a simple Sunnah, such as saying "Allahul Musta'an (Allah is the one whose help is sought)," claiming that it is not haram (forbidden)?

This phrase was uttered by the Prophet of Allah, Jacob, peace be upon him, when he was afflicted with the loss of his son Joseph, and it is mentioned in the Almighty's saying: (So beautiful patience; and Allah is the one sought for help against what you describe). Some companions, like Uthman ibn Affan, may Allah be pleased with him, also said it. Imam al-Nawawi considered it desirable to say it in such circumstances. It is not a religious obligation to utter it, but it is a good word. Whoever says it, emulating Jacob, peace be upon him, and the Companions, then it is good, and whoever remains silent, there is no sin upon him. There is ample room in this matter.