Fiqh-us-Sunnah Volume 001, Purification and Prayer, Fiqh 1.099.

Section : What Is Said During the Azhan.

It is preferred that whoever is listening to the azhan repeat it with the caller, except for the two hayya 'alas-salah, hayya 'alal-falah phrases, after which he should say La haula wa la quwatah illa billah (there is no power or might save Allah).

Says an-Nawawi, "Our companions hold that it is preferable for the listener to repeat after the caller (to prayer), except when he comes to the two preceding phrases, for this shows that he approves of what the caller is saying. Those two statements are calls to the prayer, and it is only proper for the caller to prayer to say them. It is preferable for the listener to say something, such as La haula wa la quwatah illa billah. It is confirmed in the two Sahihs from Abu Musa al-Ash'ari that the Prophet, upon whom be peace, said, 'La haula wa la quwatah illa billah is a treasure from the treasures of Paradise.' Our companions say that to repeat the call to prayer is preferred for everyone who hears the call, whether clean or unclean, in a state of post-sexual un-cleanliness or menstruating, and so on, as it is a remembrance and all of those people who can should make it. Those who can not do so are the ones who are praying, who are relieving themselves, or are having sexual intercourse. If one is reciting the Qur'an, or making remembrance of Allah (zhikr) or studying and so on, he should stop what he is doing and repeat after the caller to prayer. He may then return to what he was doing, if he wishes, or he can pray a voluntary or obligatory prayer." Says ash-Shaf'i, "One should not repeat after the call to prayer, but when he finishes he should repeat what he has said." In al-Mughni, it says, "If one enters the mosque and hears the azhan, it is best that he wait until the caller finishes it before he begins to repeat it. This way he will catch both good deeds. If he does not repeat after the call but starts praying, there is no problem. This is what Ahmad says on the subject."

The Muslim should pray for the Prophet, peace be upon him, after the call is over in any of the manners that have been related, and ask Allah to give him the place of wasilah. 'Abdullah ibn 'Amr related that the Messenger of Allah, upon whom be peace, said, "If you hear the call to prayer, repeat after it. Then supplicate for me, for whoever makes one supplication for me, Allah makes ten for him. Then ask Allah to grant me the place of wasilah. It is a place in Paradise reserved for a slave from among the slaves of Allah. I hope to be him, and whoever asks Allah to grant me the place of wasilah, my intercession becomes permissible for him." (Related by Muslim.) Jabir reported that the Prophet said, "Whoever says (after) hearing the call to prayer, 'O Allah, Lord of this complete call and of the established prayers, grant Muhammad the place of wasilah, the most virtuous place and raise him to a praiseworthy position that you have promised him,' will have my intercession made permissible for him on the Day of Judgement. (Related by al-Bukhari.)