Musnad Ahmad 26:117

Ibn Numayr narrated to us, from Mujalid, from ‘Amir, from al-Aswad ibn Yazid, who said: The prayer was established in the mosque, so we came walking with ‘Abdullah ibn Mas‘ud. When the people bowed, ‘Abdullah bowed and we bowed with him while we were walking. A man passed in front of him and said: "Peace be upon you, O Abu ‘Abd al-Rahman." ‘Abdullah said, while he was bowing: "Allah and His Messenger have spoken the truth." When he finished, some of the people asked him: "Why did you say, 'Allah and His Messenger have spoken the truth,' when the man greeted you?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Indeed, among the signs of the Hour is when the greeting is given only to those one knows.'"