Hawzah News Agency – The merciful prophet of Islam, peace be upon him and his household says:

يُسَلِّمُ الصغيرُ عَلى الكبيرِ، ويُسَلِّمُ الواحِدُ على‏ الاثنَينِ، ويُسَلِّمُ القَليلُ على‏ الكثيرِ، ويُسَلِّمُ الراكِبُ على‏ الماشي، ويُسَلِّمُ المارُّ على‏ القائمِ، ويُسَلِّمُ القائمُ على‏ القاعِدِ.

'The young should greet the old, a single individual should greet two together, the fewer number of people should greet the larger number, the person on horseback should greet the one on foot, the person walking should greet the one standing, and the person standing should greet the one sitting.'

Source: Source: Kanz al-Ummal, no. 25321