Sayyiduna Ibn Umar (may Allah be pleased with them both) narrated that Sayyiduna al-Agharr (may Allah be pleased with him), a Companion from the tribe of Muzaynah, was owed some wasqs of dates by a man from Banu Amr bin Awf. He went to him several times, and he said: "I came to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) sent Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) with me." On the way, everyone who met us greeted us with salam, so Sayyiduna Abu Bakr (may Allah be pleased with him) said: "Do you not see that people greet you with salam first and so they get the reward? Greet them with salam first, so that you get the reward."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 875