Is seeking forgiveness three times only after the obligatory prayers, or does it also include the supererogatory ones? And what is the ruling on someone who adheres to seeking forgiveness after supererogatory prayers if it is specific to obligatory ones?

Reciting istighfar (seeking forgiveness) three times after prayer applies to both obligatory and supererogatory prayers, based on Al-Sindi’s statement in his commentary on Sunan al-Nasa'i: "Seeking forgiveness after the taslim (definition) (salutation of peace) is a confirmed sunnah (definition)." Seeking forgiveness after supererogatory prayers is also recommended because the most beloved act of worship is that which is continuous, even if small. However, it must not be neglected after obligatory prayers, as supplication after an obligatory prayer is answered. The matter in this regard is broad.