Is a person sinful for abandoning remembrance (dhikr) they used to habitually perform privately after prayer, after learning that it was not mentioned in the Sunnah?

Allah has urged frequent remembrance of Him. And for one who is accustomed to a specific remembrance after prayer, he should not abandon it. If he leaves it, there is no sin upon him, but perseverance in it is recommended, due to the saying of the Prophet, peace and blessings be upon him: "The most beloved deed to Allah is that which is done regularly, even if it is small." A Muslim should add reported supplications, and if he substitutes them with something better due to his inability to combine them, there is no harm. However, restricting oneself to the reported supplications is better.