Al-Aswad bin Yazid (may Allah have mercy on him) said: The prayer was started in the mosque, so we came walking with Sayyiduna Abdullah (bin Mas'ud) (may Allah be pleased with him). When the people bowed, Sayyiduna Abdullah (may Allah be pleased with him) also bowed, and we bowed with him while we were (still) walking. Then a man passed in front of him and said: Al-salamu alayka (peace be upon you), O Abu Abd al-Rahman! Sayyiduna Abdullah (may Allah be pleased with him) said while still bowing: Allah and His Messenger have spoken the truth. When he finished (the prayer), some of the people asked him: Why did you say, when that man greeted you, that Allah and His Messenger have spoken the truth? He said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Among the signs of the Hour is that the greeting of salam is given only on the basis of acquaintance."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9458