What should we say when the muezzin says al-salaatu khayrun min al-nawm (prayer is better than sleep)?

He should be answered with something like what he says. So
one should say “al-salaatu khayrun min al-nawm”, because when the muezzin
says “Allaahu akbar (Allaah is most great)”, one should say “Allaahu akbar”,
and when he says “Ashhadu an laa ilaaha ill-Allaah (I bear witness that
there is no god but Allaah)”, one should say “Ashhadu an laa ilaaha
ill-Allaah”. And when he says “Ashhadu anna Muhammadan rasool Allaah (I bear
witness that Muhammad is the Messenger of Allaah),” one should say, “Ashhadu
anna Muhammadan rasool Allaah.” Then after saying the Shahaadatayn, one
should say: “Radaytu Billaahi rabban wa bi’l-Islaami deenan wa bi Muhammad
Rasoolan (I am content with Allaah as my Lord, and with Islam as my
religion, and with Muhammad as my Prophet).” When he says “Hayya ‘ala
al-salaah (come to prayer), one should say “Laa hawla wa laa quwwata illa
Billaah (There is no power and no strength except with Allaah),” and the
same when he says “Hayya ‘ala al-falaah (come to prosperity).” When he says
“Allaahu akbar”, one should say “Allaahu akbar,” and when he says “Laa
ilaaha ill-Allaah” one should say “Laa ilaaha ill-Allaah.” And when he says
“Al-salaatu khayrun min al-nawm”, one should say “al-salaatu khayrun min
al-nawm.” 

It was said that he should say “Sadaqta wa barirta (You have
spoken the truth); and it was said that he should say “Laa hawla wa laa
quwwata illa Billaah.” But the correct view is the first one. The evidence
for that is the words of the Prophet (peace and blessings of Allaah be
upon him): ‘When you hear the muezzin, say what the muezzin says.” There is
no exception to this in the Sunnah except in the case of the words “Hayya
‘ala al-salaah, hayya ‘ala al-falaah,” when it should be said, “Laa hawla wa
laa quwwata illa Billaah.” The general meaning of the hadeeth remains
applicable to everything except these two phrases. 

If someone were to ask: Isn’t saying “prayer is better than
sleep” true? The answer is, yes indeed, and saying Allaah akbar is true, and
saying Laa ilaaha ill-Allaah is true, so when he says Allaah akbar, should
you say Sadaqta wa barirta? You should not say that, so when he says “al-salaatu khayrun min al-nawm,” say what he says. This is the general meaning of the command of the Prophet (peace and blessings of Allaah be
upon him). End quote.