It is narrated from Sayyiduna al-Hasan bin Ali (may Allah be pleased with him) that it was said: "O Messenger of Allah, some people come to a house and one of them asks permission to enter; does that suffice for all of them?" He said: "Yes." It was said: "And if one man among them responds, does that suffice for all of them?" He said: "Yes." It was said: "Some people pass by and one of them gives the greeting of peace (salam); does that suffice for all of them?" He said: "Yes." It was said: "And if one man among them returns the greeting, does that suffice for all of them?" He said: "Yes."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2717