smile is Sadaqah

تَبَسُّمُكَ فِي وَجْهِ أَخِيكَ لَكَ صَدَقَةٌ

“Smiling in your brother's face is an act of charity.”

—Prophet Muhammad ﷺ

source: Jami at-Tirmidhi 1956

https://sunnah.com/urn/672590