The young should salute the old, the one who is passing by should salute the one who is sitting, and a small company should salute a large one.  

قَالَ رَسُولُ اللَّهِ ﷺ يُسَلِّمُ الصَّغِيرُ عَلَى الْكَبِيرِ وَالْمَارُّ عَلَى الْقَاعِدِ وَالْقَلِيلُ عَلَى الْكَثِيرِ  

one who is riding should salute one who is walking. He then mentioned the rest of the tradition.  

قَالَ رَسُولُ اللَّهِ ﷺ يُسَلِّمُ الرَّاكِبُ عَلَى الْمَاشِي ثُمَّ ذَكَرَ الْحَدِيثَ