«إذا مر رجال بقوم فسلم رجل من الذين مروا على الجلوس ورد من هؤلاء واحد أجزأ عن هؤلاء وعن هؤلاء»
The Prophet (peace and blessings be upon him) said:
"When some men pass by a group of people and one of those passing greets those sitting with salam, and one of these (sitting) returns the greeting, it suffices on behalf of these (passing) and on behalf of these (sitting)."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 798