Muwatta Malik, Book 53, Hadith 1 (USC-MSA) ⓘ
Yahya related to me from Malik from Zayd ibn Aslam that the Messenger of Allah, may Allah bless him and grant him peace, said, "The one riding greets the one walking, and when one of a group of people gives a greeting, it is enough for all of them."
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 " .
also referenced under: Book 53, Hadith 1759 (In-Book)