Adhan

عَنْ أَبِي سَعِيدٍ الخُدْرِيِّ رضي الله عنه أَنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ: «إِذَا سَمِعْتُمُ النِّدَاءَ، فَقُولُوا مِثْلَ مَا يَقُولُ المُؤَذِّنُ».

The Prophet (may Allah's peace and blessings be upon him) is urging us here to respond to the Muezzin on hearing him by repeating what he says word for word. So, when he says: "Allāhu Akbar" (Allah is the Greatest), we should say: "Allāhu Akbar" after him; and when he pronounces the two testimonies of faith, we should pronounce them after him. The only exception is when he says: "Hayy ‘ala as-salāh, hayy ‘ala al-falāh" (Come to prayer, come to success), we should say: "La hawla wala quwwata illa billāh" (There is no might or power except through Allah).

• After the first Muezzin finishes the Adhan, one should repeat after the second one, even if there are multiple Muezzins, given the general indication of the Hadīth.
• One should respond to the Muezzin in all situations unless he is in the bathroom or answering the call of nature.

صحيح البخاري (1/ 126) (611). صحيح مسلم (1/ 288) (383). تيسير العلام شرح عمدة الأحكام، للبسام (ص120). الإلمام بشرح عمدة الأحكام، لإسماعيل الأنصاري (1/ 98).

عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ رضي الله عنهما أَنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ: «مَنْ قَالَ حِينَ يَسْمَعُ النِّدَاءَ: اللَّهُمَّ رَبَّ هَذِهِ الدَّعْوَةِ التَّامَّةِ، وَالصَّلاَةِ القَائِمَةِ، آتِ مُحَمَّدًا الوَسِيلَةَ وَالفَضِيلَةَ، وَابْعَثْهُ مَقَامًا مَحْمُودًا الَّذِي وَعَدْتَهُ، حَلَّتْ لَهُ شَفَاعَتِي يَوْمَ القِيَامَةِ».

The Prophet (may Allah's peace and blessings be upon him) clarifies that whoever says the following upon hearing the muezzin completing the Adhān (call to prayer): (O Allah, Lord of this perfect call), these are the words of the Adhān that are used to call people to worship Allah and to perform the prayer; (perfect) complete, i.e., the call of Tawhīd (monotheism) and the divine message. (And the prayer that is to be established) constantly and that is about to be offered. (Grant) and give (Muhammad Al-Wasīlah) and the highest position in Paradise that is not to be granted to anyone other than him (may Allah's peace and blessings be upon him), (and Al-Fadīlah), i.e., the degree of superiority over others' degrees, (and resurrect him to) and give him (the Praiseworthy Station) that brings praise to the one standing therein, which is a reference to the Grand Intercession on the Day of Judgment (that You have promised him) by saying: {So your Lord may raise you to a praised status} to be yours. Whoever says this supplication becomes worthy of the Prophet's intercession that will be definitely granted to him.

• It is a Shariah directive to say this supplication after repeating the words of the Adhān after the muezzin, but whoever does not hear the call does not say it.
• The merit of the Prophet (may Allah's peace and blessings be upon him) as he was granted the highest position in Paradise, the degree of superiority, the praiseworthy station, and the Grand Intercession in judging between people.
• Establishing the intercession for the Messenger of Allah (may Allah's peace and blessings be upon him), as indicated by his saying: "He will be granted my intercession on the Day of Judgment."
• The Prophet's intercession will be for those who committed major sins from his Ummah so as not to enter Hellfire, or for those who have already entered it to be taken out, or to be admitted to Paradise without reckoning, or for those who entered Paradise to have their degrees raised.
• At-Tībi said: From its beginning until saying "Muhammad is the Messenger of Allah" is the perfect call. The "Hy‘alah" (saying: Hasten to prayer, hasten to success) refers to the established prayer as in His statement: {establish prayer}. It is probable that "prayer" means supplication and "established" means constant, and based on this, his saying "the prayer that is to be established" is a clarification of the perfect call. It is possible that the intended meaning of the "prayer" referred to here is the specific, established prayer being called to at that time, which is more likely to be correct.
• Al-Muhallab said: The Hadīth encourages supplication at the times of prayer because it is a situation when supplications are hoped to be answered.

صحيح البخاري (1/ 126) (614). توضيح الأحكام مِن بلوغ المرام، لعبد الله البسام (1/ 550). فتح ذي الجلال والإكرام شرح بلوغ المرام، لابن عثيمين (1/ 488). شرح سنن أبي داود، لعبد المحسن العباد (73/ 18).