How should one respond when the mu’adhdhin says “As-salatu khayrun min an-nawm (prayer is better than sleep)”?
Question

What should be said after the mu’adhdhin says “As-salatu khayrun min an-nawm (prayer is better than sleep)”? Please provide evidence for that. May Allah reward you with good.
Praise be to Allah.It is recommended (mustahabb) for the one who hears the adhan to say what the mu’adhdhin says, except when he says “Hayya ‘ala as-salah (come to prayer)” and “Hayya ‘ala al-falah (come to success),” in which case it is recommended to say, “La hawla wa la quwwata illa Billah (There is no power and no strength except with Allah).”
The evidence for that is the words of the Prophet (blessings and peace of Allah be upon him):
“When you hear the mu’adhdhin, say what he says.” Narrated by al-Bukhari (611) and Muslim (318).
Ibn Qudamah said in al-Mughni (1/591):
I do not know of any difference of opinion among the scholars regarding the fact that this is recommended. End quote.
That includes when the mu’adhdhin says in the call to Fajr prayer: “As-salatu khayrun min an-nawm (prayer is better than sleep).” In this case it is recommended for the one who hears it to repeat the same, so he should say, “As-salatu khayrun min an-nawm.”
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said in ash-Sharh al-Mumti‘ (2/84):
The correct view is that he should say the same as the mu’adhdhin says, “(prayer As-salatu khayrun min an-nawm,” because the Prophet (blessings and peace of Allah be upon him) said: “When you hear the mu’adhdhin, say what he says.” Narrated by al-Bukhari (611) and Muslim (318).
This was the view of some of the Maliki jurists. See: al-Mawsu‘ah al-Fiqhiyyah, 2/372.
With regard to what some jurists regarded as recommended, which is to say, “Sadaqta wa bararta (you have spoken the truth and done well),” there is no evidence for that and it is contrary to the general meaning of the hadith quoted above, “say what he says.” The basic principle is that acts of worship are disallowed unless evidence for them is proven.
Al-Hafiz Ibn Hajar (may Allah have mercy on him) said in at-Talkhis al-Habir (1/378):
There is no basis for what is mentioned about the response to “As-salatu khayrun min an-nawm (prayer is better than sleep).” End quote.
As-San‘ani (may Allah have mercy on him) said in Subul as-Salaam (1/190):
It was said that in response to the phrase “As-salatu khayrun min an-nawm,” one should say, “Sadaqta wa bararta (you have spoken the truth and done well).” This is something that is thought to be a good idea by those who say it, but there is no sound report from the Prophet (blessings and peace of Allah be upon him) to prove that. End quote.
Shaykh Muhammad ibn Ibrahim said in Majmu‘ al-Fatawa (2/no. 448):
The words of the Prophet (blessings and peace of Allah be upon him), “say what he says,” indicate that one should repeat, “As-salatu khayrun min an-nawm (prayer is better than sleep).” As for saying, “Sadaqta wa bararta (you have spoken the truth and done well),” that is only mentioned in a weak (da‘if) hadith. Hence the one who wants to say something you should choose to say, “As-salatu khayrun min an-nawm.” The correct view – and Allah knows best – is that he should not respond by saying, “Sadaqta wa bararta.” I hear some people combining the two phrases, so they say, “As-salatu khayrun min an-nawm; sadaqta wa bararta (prayer is better than sleep; you have spoken the truth and done well).” But there is no basis for that; rather it is better to look at the evidence. End quote.
And Allah knows best.