Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Three men came to the Prophet (peace and blessings be upon him). The first said: "As-salamu alaykum," and the Prophet (peace and blessings be upon him) replied: "Wa alaykum wa rahmatullah." Then the second came and said: "As-salamu alaykum wa rahmatullah," and the Prophet (peace and blessings be upon him) replied: "Wa alaykum wa rahmatullahi wa barakatuh." Then the third came and said: "As-salamu alaykum wa rahmatullahi wa barakatuh," and the Prophet (peace and blessings be upon him) replied: "Wa alaykum." The father of that young man was sitting with the Prophet (peace and blessings be upon him), and he said: "O Messenger of Allah! You gave more (words) in reply to so-and-so and so-and-so, but you did not give my son anything more." The Messenger of Allah (peace and blessings be upon him) said: "We found no room to add to his greeting, so we returned to him the same as what he had said."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11957