Abu Hurayra reported that the Prophet, may Allah bless him and grant him peace, said, "When a man comes to a gathering, he should give the greeting. When he sits down and then thinks that he should leave before the gathering has broken up, he should give the greeting. Neither is more of a duty than the other."  

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا جَاءَ الرَّجُلُ الْمَجْلِسَ فَلْيُسَلِّمْ فَإِنْ جَلَسَ ثُمَّ بَدَا لَهُ أَنْ يَقُومَ قَبْلَ أَنْ يَتَفَرَّقَ الْمَجْلِسُ فَلْيُسَلِّمْ فَإِنَّ الأُولَى لَيْسَتْ بِأَحَقَّ مِنَ الأخْرَى