Abu Hurairah (may Allah be pleased with him) narrates from the Prophet (peace and blessings of Allah be upon him), he said: "When any one of you meets his (Muslim) brother, he should greet him with Salam. If a tree, wall, or stone comes between them and then they meet again, he should greet him with Salam again." Its chain is authentic, narrated by Abu Dawood.
Hadith Referenceمشكوة المصابيح / كتاب الآداب / 4650
Hadith Gradingالألبانی:صَحِيح | زبیر علی زئی:إسناده صحيح
Hadith Takhrij´تحقيق و تخريج: محدث العصر حافظ زبير على زئي رحمه الله` «إسناده صحيح، رواه أبو داود (5200)»