حَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ يُسَلِّمُ الرَّاكِبُ عَلَى الْمَاشِي وَإِذَا سَلَّمَ مِنَ الْقَوْمِ وَاحِدٌ أَجْزَأَ عَنْهُمْ ‏"‏ ‏.‏

Yahya related to me from Malik from Zayd ibn Aslam that theMessenger of Allah, may Allah bless him and grant him peace, said,"The one riding greets the one walking, and when one of a group ofpeople gives a greeting, it is enough for all of them." Translation Not Available

Book reference: Muwatta Imam Malik Book 53 Hadith 1759
Web reference: Muwatta Imam Malik Book 53 Hadith 1