The following Dua should be recited when seeing a Muslim smile,
أَضْحَكَ اللہُ سِنَّكَ
May Allah always keep you smiling.
(Bukhari #3294)
Sayyiduna ‘Abdullah ibn ‘Abbas (Radiyallahu’anhu) reports that Rasulullah (sallallahu alayhi wasallam) said, “Blessing (barakah) is with your elders/ seniors.” Explanation: Elders refers to those who are experienced and punctual on good actions. Remain in their company so that you can follow their advices and attain guidance through emulating their ways. (Faydul Qadir, Hadith :3205)

The following Dua should be recited when seeing a Muslim smile,
أَضْحَكَ اللہُ سِنَّكَ
May Allah always keep you smiling.
(Bukhari #3294)