عَنْ أَبِى هُرَيْرَةَ قَالَ إِذَا لَقِىَ أَحَدُكُمْ أَخَاهُ فَلْيُسَلِّمْ عَلَيْهِ فَإِنْ حَالَتْ بَيْنَهُمَا شَجَرَةٌ أَوْ جِدَارٌ أَوْ حَجَرٌ ثُمَّ لَقِيَهُ فَلْيُسَلِّمْ عَلَيْهِ أَيْضًا.
Abu Hurairah (Radi Allahu anhu) said: “Whenever one of you meets his brother, he should give him salams. If a tree, a wall or a rock comes between them, then he meets him again on the other side he should greet him again as well.”
Collected by Abu Dawud (5200) Graded as being Sahih by Shaykh Al-Albani
Benefits for this topic:
- Imam An-Nawawi titled a chapter in Riyadus Salaheen, ” The recommendation to greet the acquaintance repeatedly.”
- If a person is at work, at school or, so forth and often passes by his brother, he should give him the salams. Some people only greet the first time they see each other on that day at school or at work. This is in opposition to the Sunnah. 
Description for this Sunnah
- If the companions were walking and there was tree in the road, some of them walked on the right side of the tree and others on the left side. When they met up again they would give salams to each other once more.
- The man who prayed incorrectly tried to pray three times. After each time he returned to the Prophet (sallAllahu alayhi wa salam) and gave him the greetings.
Why revive this Sunnah?
- If a Muslim meets his brother and gives him salams, then sees him again shortly and gives the greeting again, this increases love, brotherhood and he earns a reward.
- As-Salamu Alaykum : is a supplication for blessings. When you say to your brother, ” As-Salamu alaykum.” This means you ask Allah to grant him safety and security. 
- As-Salamu alaykum is the Muslims’ slogan.
 Al-‘Illam bi ba’d Ahkam As-salam by Shaykh Abdus Salam Burjus(May Allah have mercy on him) page 69, printed by Daru Istaqamah 1st edition 2009.
 At-Tabarani in Awsot # 8153
 Collected by Bukhari (#6667 ) and Muslim (# 911)
 Al-‘Illam bi ba’d Ahkam As-salam
 Awnul Ahadis Samad sharh Al-Adabu Al-Mufrad by Shaykh Zayd Madkhali vol 3/ page 157, printed by Mirath An-Nabuwa 1434H, 1stedition.