When seeing a Muslim happy:
أَضْحَكَ اللَّهُ سِنَّكَ
“May Allah fill your life with laughter.”
This short du’a serves as a reminder for us to not only be in a state of happiness, but to also wish happiness upon one another. Throughout life, we may have many occasions where hardships come upon us, but when everything is put into perspective, happiness should be a dominant emotion; we have Allah ﷻ, the Prophet ﷺ, and the blessing of the deen in our lives. These three are what make true happiness, because they will carry us over to eternal happiness. Furthermore, it is learned from this Hadith that one must remain positive, and look at the positive in others as well, which results in healthy relationships.
