الحمد لله رب العالمين رحمكم الله أجمعين
al-Ḥamdu li-llāhi rabbi l-ʿālamīn… Raḥimakum Allāh… Ajmaʿīn
The exchange after a sneeze: the sneezer praises God, those present answer with a prayer for mercy, and the sneezer replies with a prayer for all.
What it means
When a Muslim sneezes, he or she says al-ḥamdu li-llāhi rabbi l-ʿālamīn, “praise belongs to God, Lord of the worlds”. Those present answer raḥimakum Allāh, “may God have mercy on you”, and the one who sneezed replies with a prayer that includes everyone, ajmaʿīn, “all of us together”. Sahih al-Bukhari reports from Abu Hurayra that the Prophet taught this courtesy: the sneezer says al-ḥamdu li-llāh, the brother answers yarḥamuka llāh, and the sneezer replies yahdīkumu llāhu wa yuṣliḥu bālakum, “may God guide you and set your affairs right”. The wording of the reply varies, and the exchange binds believers in mutual good will.
Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 63, listed as AL ḤAMDU LILLAH RABB AL ‘ĀLAMĪN… RAḤIMAKUM ALLAH… AJMA‘ĪN.. The definition is restated here.