Assalaamu alaykum. When one does not have a legitimate excuse, would it be contrary to good manners to leave the masjid immediately after the tasleem of a congregational prayer?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

It was reported that people used to leave the mosque immediately after the end of the prayer during the lifetime of the Prophet, sallallaahu ʻalayhi wa sallam. Abu Hurayrah narrated the incident in which the Prophet, sallallaahu ʻalayhi wa sallam, forgetfully recited tasleem before finishing the prayer. He said, “Those who were in a hurry (as-saraʻaan) left the mosque and said, ‘Has the prayer been shortened?!’” [Al-Bukhari and Muslim]

The hadeeth refers to the worshipers who rush to leave the mosque right after the end of the prayer. An-Nihaayah (book) reads, “The Arabic word as-saraʻaan (in the hadeeth) refers to those who hurriedly hasten towards something and quickly race towards it...”

There is no doubt that hastening to leave the mosque right after the prayer entails missing great rewards that one could have earned; the angels keep invoking blessings on the worshiper as long as he remains in the place where he prayed. He might also miss the rewards earned by reciting the reported thikr (remembrance of Allaah) and the supplications after the prayers: tasbeeh (saying subhaan-Allaah), tahmeed (saying alhamdulillaah), takbeer (saying Allaahu Akbar), tahleel (saying Laa ilaaha illa Allaah), and reciting Al-Kursi verse (Quran 2:255). Please, refer to fatwa 275431.

It should be noted that one can still recite such reported thikr and supplications after leaving the mosque. Ibn ʻUthaymeen wrote, “If the praying person broke his ablution after performing the prayer, he can still recite the reported thikr after the prayers because reciting thikr does not necessitate being in a state of ritual purity ... Similarly, if he leaves the mosque after the prayer, he can still recite the reported thikr that is recited after the prayer...”

Allaah knows best.