Ibn Majah

Benefits and Issues:
(1)
If a few people are sitting together in the mosque, then the one who comes to them should greet them with salam.

(2)
It is necessary to reply to the salam.

(3)
«عليك» is for one person and «عليكم» is for more than one person, but it is also correct to say «عليكم» to a single person.

Commentary:
(1)
The chain of narration of this hadith is weak; however, an incident of this type is also narrated through another authentic chain. In Sahih al-Bukhari, it is narrated from Abu Juhaym ibn Harith radi Allahu anhu that he said: The Prophet sallallahu alayhi wa sallam was coming from the direction of Bi’r Jamal when a man met him. He greeted (with salam), so the Prophet sallallahu alayhi wa sallam, before replying to the greeting, struck his hand on the wall and wiped his face and hands (performed tayammum), then replied to the greeting. (Sahih al-Bukhari, Kitab al-Tayammum, Chapter: Tayammum in Residence if Water is Not Found and One Fears Missing the Prayer, Hadith: 337)

(2)
It is permissible to delay replying to a greeting due to an excuse or preoccupation.

(3)
Imam al-Bukhari rahimahullah has deduced from the aforementioned incident that travel is not a condition for tayammum. From the apparent wording of Surah al-Ma’idah, verse 6, it seems that tayammum can only be performed in the state of travel. From this hadith, it is understood that the verse mentions those circumstances in which, generally, the need for tayammum may arise; it does not mean that tayammum is not permissible outside of those circumstances.