From the etiquette of attending the Masaajid is that that one should not leave the Masjid after the adhaan is called except due to a valid excuse. The reason for this is that leaving the Masjid is to leave off that which the adhaan itself calls to (which is requesting the men to attend the Masjid to fulfill the congregational prayer) - so leaving the Masjid after the adhaan becomes a reason for busying oneself away from praying or praying late.
Abu Hurairah (radhi Allaahu 'anhu) stated that the Prophet (salallaahu 'alaihi wassallam) said: "When the adhaan is called for the prayer, the shaitaan takes to his heels and flees and passes wind so that he he may not hear the adhaan..." (Bukhaaree, 583. Muslim, 389).
Ibn Battaal (rahimahullaah): "It resembles the rebuke of exiting of a person from the Masjid after the mu'adhin has made the adhaan [taken] from this meaning, so as not to resemble the Shaytaan who flees upon hearing the adhaan - and Allaah knows best." (Sharh Ibn Battaal 'alal-Bukhaaree, 2/235. Fathul-Baaree 2/87)
Abu Sha'thaa stated: We were sitting in the Masjid with Abu Hurairah (radhi Allaahu 'anhu) so the Mu'adhin called the adhaan - a man then stood up and began to walk out from the Masjid. So Abu Hurairah followed him with his eyes until he exited the Masjid. So Abu Hurairah said: "As for this one, then he has disobeyed Abul-Qaasim (salallaahu 'alaihi wassallam)." (Muslim, 389. See I'laam al-Muwaqqi'een, 3/160)
And this is marfoo' (i.e. raised and ascribed to the Messenger) in its ruling, and is therefore used as a proof. This is because the ruling upon an issue being obedience or disobedience is not established except by a revealed text from the Legislator (Allaah). And a Sahaabee does not make an affair binding except that he has knowledge of it. It is reported from Abu Hurairah (radhi Allaah 'anhu) that he said: Allaah's Messenger (salallaahu 'alaihi wassallam): "There is not a person who hears the call to prayer in this Masjid of mine, then he leaves from it, except due to a need, and then does not return to it except he be a Munaafiq." (Tabaraanee in al-Awsat 4/501,502. Authenticated by al-Albaanee in Saheeh at-Targheeb, 1/179)
At-Tirmidhee (radhi Allaahu 'anhu) stated after mentioning the hadeeth of Abu Hurayrah above: "And the People of Knowledge from amongst the Sahaabah (radhi Allaahu 'anhum) and those who followed them acted upon this - not one of them would leave the Masjid after the adhaan except due to a valid excuse [such as] if he is not in a state of wudhoo or a necessary affair." (Jaami' at-Tirmidhee, 1/398).
As for the one who leaves the Masjid due to a valid excuse such as for the purpose of performing wudhoo, then there is evidence to show that. The following hadeeth of Abu Hurairah (radhi Allaahu 'anhu) indicates this: "The Prophet (salallaahu 'alaihi wassallam) came out and the iqaamah was pronounced and the rows straightened. The Prophet (salallaahu 'alaihi wassallam) stood in his prayer place and we waited for the Prophet (salallaahu 'alaihi wassallam) to begin with the takbeer. He turned away [leaving] and said "Remain in you places". We kept on standing till the Prophet (salallaahu 'alaihi wassallam) returned and the water was trickling from his head as he had taken a bath [of Janaabah]." (Bukhaaree, 639. Muslim, 605).
So likewise in this manner it is allowed for the Imaam or the Mu'adhin to leave and go to another Masjid to attend a class (dars) or similar to that because this is a legislated reason which enters into the generality of texts that allow exceptions.
It is reported from 'Uthmaan bin 'Affaan (radhi Allaahu 'anhu) that he said that Allaah's Messenger (salallaahu 'alaihi wassallam) said:
"Whoever witnesses the adhaan in the Masjid and then leaves, not leaving leaving due to any need, and he does not intend to return, then he is a hypocrite." (Reported by Ibn Maajah. It is stated in az-Zawaa'id, 1/259: "It contains [in the isnaad] Ibn Abee Farwah and he is da'eef and likewise Abdul-Jabbaar bin 'Umar..." Al-Albaanee authenticated it in Saheeh at-Targheeb, 1/179. And he declared it da'eef in Da'eef al-Jaami', 5/156)
Sa'eed bin al-Musayyib stated that the Prophet (salallaahu 'alaihi wassallam) said: "No one exits the Masjid after the call to prayer (i.e. the adhaan) except for a hypocrite, unless it be one who leaves for a need and he intends to return [to the Masjid]." (Reported by Abu Dawood in al-Maraaseel, p. 131. And Abdur-Razzaaq, 1/508, from Sa'eed with this meaning. Al-Albaanee authenticated it in Saheeh at-Targheeb, 1/179, see as-Saheehah, 2518)
Shaikh al-Albaanee (rahimahullaah) said: "This action is the action of a munaafiq, since this is not the state of the believer in truth - so the nifaaq (hypocrisy) here is nifaaq in action, and not in belief and Allaah knows best." (Saheeh at-Targheeb)
Some of the Mu'adhins when they call the adhaan they leave the Masjid and go to their homes or attend to other business, even if he intends to return back - however it is better especially in his case not to leave the Masjid, because it is feared that his leaving is not due to any valid need, rather it is something from habit or due to boredom of sitting in the Masjid until the iqaamah - especially if the time periods between the adhaan and the iqaamah are lengthy as is the vase with most Masaajid. Further the Mu'adhin is more worthy of being an example than others because he calls people to attend the Prayer and to attend early - then it is not correct that he leaves and does not return himself except just before the iqaamah. So it is necessary to pay attention to this and for him to take note.
Abu Khadeejah Abdul-Waahid, Birmingham, UK.
[Benefits taken from: Ahkaam Hudhoor il-Masaajid, with additions]
http://www.salafitalk.com/threads/254-Leaving-the-Masjid-after-the-Adhaan-is-Called
