Musnad Ahmed: Hadith 3640

The iqamah for prayer was given in the mosque and we came walking with ‘Abdullah bin Mas'ood (رضي الله عنه). When the people bowed, 'Abdullah bowed and we bowed with him whilst we were still walking. A man passed in front of him and said: Assalamu 'alaika ya Aba 'Abdur-Rahman. ‘Abdullah said whilst he was bowing. Allah and His Messenger spoke the truth. When he had finished, some of the people asked him. Why did you say, when the man greeted you, Allah and His Messenger spoke the truth? He said: I heard the Messenger of Allah (ﷺ) say: “One of the portents of the Hour will be when greetings are restricted to people whom one knows."