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:
Responding to the question, Dr. Sano Koutoub Moustapha, Professor of jurisprudence and its principles at the International Islamic University, Malaysia, states:
It is a noble intention to sustain fasting as a habit after Ramadan. However, you might know that there are specific fastings that are recommended after Ramadan such as six days of Shawwal, three days every month (11, 12, 13 or 13, 14, 15), and Mondays and Thursdays throughtout the year.
You may consider performing one of these recommended fastings 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.
Allah Almighty knows best.