Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Observing voluntary fasting after Ramadan helps one come closer to Allah. Fasting is very pleasing to Allah and one of the means for gaining abundant rewards and having sins forgiven.
Responding to your question, Dr. Sano Koutoub Moustapha, deputy chairman of the International Fiqh Academy and professor of Fiqh and its principles at the International Islamic University, Malaysia, states,
May Allah bless you with more strength and power. Indeed, it is a noble intention to sustain fasting as a habit after Ramadan. However, you might know that there are specific fasts that are recommended after Ramadan such as six days of Shawwal, three days every month (13, 14, and 15), and Mondays and Thursdays throughout the year.
You may consider performing one of these recommended fasts though we are allowed to fast without following a specific nafl recommended by Prophet Muhammad (peace and blessings be upon him).
Finally, do seek the assistance of Almighty Allah in strengthening your faith and helping you to uphold His commands and duties.
May Allah bless you and accept all your endeavors.
Allah Almighty knows best.