Prayer Is Better than Sleep: First or Second Adhan?
Question

With regard to the words as-salatu khayrun min al-nawm (prayer is better than sleep), is it to be said in the first adhan before Fajr, or in the second adhan? What is the evidence for saying them? What should the one who hears them say after the muadhdhin?
Praise be to Allah.When ‘Prayer Is Better than Sleep’ should be said
The Sunnah is to say these words “as-salatu khayrun min al-nawm (prayer is better than sleep)” in the second adhan after dawn has broken. It is narrated in the hadith of Abu Mahdhurah and the hadith of ‘Aishah (may Allah be pleased with her) that the muadhdhin used to say them in the second adhan after dawn broke.
‘Aishah said: Then the Prophet (blessings and peace of Allah be upon him) would get up and pray two rak‘ahs, then he would go out to pray after the first call, which was the second adhan in relation to what is called the first adhan , and is the first adhan with regard to the iqamah, because it may also be called “adhan.” 
The Sunnah is to say these words in this adhan which is the second one after dawn has broken, and is the first in relation to the iqamah . With regard to the first adhan, as people customarily call it, this is to alert people. The Prophet (blessings and peace of Allah be upon him) said concerning it: “So that those of you who are praying qiyam can rest and those of you who are sleeping can wake up.” 
This is the adhan that alerts people so that those who are asleep can wake up and those who are praying qiyam can rest, i.e., not make their prayers too long, because dawn has come. In the hadith of ‘Aishah, the later adhan clearly called the first adhan in relation to the iqamah, because the iqama is a second adhan, and the later adhan may be called the second adhan, in relation to the first adhan , the purpose of which is to alert people. 
Some of the scholars are of the view that these words should be said in the first adhan which serves to alert people before dawn breaks. This issue is broad in scope, in sha Allah, but these words should not be said in both calls. It is better to say them in the second call which is the first in relation to the iqamah and is the adhan that is given after dawn breaks. 
The meaning of ‘prayer is better than sleep’
What is meant by the words “prayer is better than sleep” is that the obligatory prayer that Allah has enjoined is better than sleep, and what people should do is get up for it. With regard to the nafl prayers at the end of the night or during the night, they are not obligatory, and sleep may be better than prayer if being sleepy is going to affect his prayer. [In that case] he should sleep and have his share of rest so that he will be able to pray properly. But the obligatory prayer is something that is required and is better than sleep in all cases; the individual has to get up for it and do what will help him to perform the prayer mindfully and do it well and properly. 
Should we repeat ‘prayer is better than sleep’?
The one who hears this phrase should repeat it, “al-salatu khayrun min al-nawm (prayer is better than sleep)” because the Prophet (blessings and peace of Allah be upon him) said: “When you hear the muadhdhin, say what he says.” So the one who is responding should say “Al-salatu khayrun min al-nawm”, just as he repeats “Allahu akbar (Allah is most great)” and “Ashhadu an la ilaha ill-Allah (I bear witness that there is no God but Allah)”; this is the same, he should say “al-salatu khayrun min al-nawm.” But when the muadhdhin says “Hayya ‘ala al-salah, hayya ‘ala al-falah (come to prayer, come to prosperity)”, he should say, “La hawla wa la quwwata illa Billah (there is no power and no strength except with Allah).” This is what is prescribed. 
When the Prophet (blessings and peace of Allah be upon him) heard the muadhdhin say “Hayya ‘ala al-salah (come to prayer)”, he would say, “La hawla wa la quwwata illa Billah (there is no power and no strength except with Allah).” When the muadhdhin says, “Hayya ‘ala al-falah” say “La hawla wa la quwwata illa Billah ,” because no one knows whether he will have strength or not, or whether that will be easy for him or not, so he should say La hawla wa la quwwata illa Billah, meaning I do not have the power to respond to the muadhdhin and come to the mosque and perform the prayer except by the help of Allah, may He be glorified and exalted; and I do not have the strength to do that except by the help of Allah, may He be glorified and exalted. 
The call of the muadhdhin is a call to good: “Hayya ‘ala al-salah, hayya ‘ala al-falah (Come to prayer, come to prosperity).” So he has to respond and say, “La hawla wa la quwwata illa Billah (There is no power and no strength except with Allah).” This is what is prescribed, meaning: I have no power or strength to respond to the muadhdhin or perform the prayer on time with the congregation, or to do anything, except with the help of Allah, may He be glorified and exalted.” (Shaykh ‘Abd al-‘Aziz ibn Baz (may Allah have mercy on him), from Fatwa Nur `ala al-Darb, 2:685-7) 
And Allah knows best.