Ibn Majah

37. Commentary:

➊ It is not appropriate to remember Allah in a state of impurity, therefore, it is not correct to reply to greetings (salam) or to engage in remembrance (dhikr) while urinating or defecating. When one cannot reply, then one should also not offer salam to such a person. In other words, in any state where replying to salam is prohibited, initiating salam is also not correct—except in the case of prayer (salah), where it is Sunnah to reply to salam with a hand gesture.

➋ The scholars (ahl al-'ilm) state: Just as it is not correct to reply to salam while relieving oneself, likewise, it is not correct in this state to reply to someone who sneezes, to say "alhamdulillah" oneself, or to reply to the adhan. Similarly, during the state of sexual intercourse (jima‘), one should refrain from these matters as well.

Benefits:

A Muslim person does not respond to greetings (salam) while in the state of relieving himself. There is consensus among all scholars on this matter. The Shafi'is state: It is disliked (makruh) to greet a person who is engaged in relieving himself, and if someone greets a person who is sitting for relieving himself, then it is disliked (makruh) for him to reply to the greeting. Furthermore, a person sitting for relieving himself should neither engage in remembrance (dhikr) nor recite tasbih (glorification) or tahlil (declaration of the oneness of Allah). He should neither reply to a greeting, nor respond to someone who sneezes, nor say «الحمد لله» in the event of sneezing, nor should he respond to the call to prayer (adhan) of the muezzin. Similarly, engaging in the aforementioned remembrances and supplications is disliked (makruh) during the state of sexual intercourse (jima‘), and in such states, if one sneezes, «الحمد لله» should be said in the heart and the tongue should not be moved.

[نووي: 64/4]

English Commentary:
1:
And the preferred opinion is that the Prophet (sallallahu alayhi wa sallam) did not reply because he was in the state of urination,
not because it is impermissible to reply to the greeting (salam) without ablution (wudu).
And the narrations in which it is mentioned that “the Prophet (sallallahu alayhi wa sallam) performed ablution after finishing and then replied” are to be understood as indicating recommendation (istihbab),
and furthermore, it was especially beloved to him that he would not mention the name of Allah without being in a state of ritual purity (taharah).
Another point established from this hadith is that one should not give salam to a person who is relieving himself (defecating or urinating),
and this ruling is obligatory.

331. Commentary:
Out of the two aforementioned narrations, the narration mentioning striking the hands twice is rejected (munkar) and weak, while the narration mentioning striking the hands once is authentic (sahih); therefore, the practicable hadith is only this one.

Hadith Commentary: Benefits and Issues: From other narrations, it is understood that he gave salam after relieving himself, but the Prophet (sallallahu alayhi wa sallam) did not reply due to the lack of purity. To give salam or reply to it while in a state of urination or defecation is contrary to etiquette and good manners. According to Imam Abu Hanifah, the muezzin, the one praying, the reciter (of the Qur'an), and the preacher are not obligated to reply to salam; they may respond in their heart. According to Imam Muhammad, one should reply after completing (the act). And according to Imam Abu Yusuf, he will not reply.