Hadith 6:987
Yahya ibn Adam narrated to us, from Zuhayr, from Musa ibn Uqbah, that he saw Salim ibn Abdullah—he would not pass by a grave, neither by night nor by day, except that he would greet it while we were traveling with him, saying: "Peace be upon you." I asked him about that, and he informed me, on the authority of his father, that he used to do that.
حدثنا يحيى بن آدم عن زهير عن موسى بن عقبة أنه رأى سالم بن عبد الله لا يمر بليل ولا نهار بقبر إلا يسلم عليه ونحن مسافرون معه يقول السلام عليكم فقلت له في ذلك فأخبرنيه عن أبيه أنه كان يصنع ذلك