If he recites “Is not Allaah the Best of judges?” in the prayer, should he say, “Bala (Yes indeed)?

Firstly: 

Speaking during prayer was permitted. The Prophet (peace
and blessings of Allaah be upon him) and his companions used to return
salaams when they were praying. Then that was forbidden, and the scholars
are unanimously agreed that it is forbidden to speak during the prayer and
the prayer is invalidated if one does that knowingly and deliberately.  

It was narrated that ‘Abd-Allaah ibn Mas’ood (may Allaah be
pleased with him) said: We used to greet the Messenger of Allaah (peace
and blessings of Allaah be upon him) when he was praying, and he would
return the greeting. When we came back from being with the Negus, we greeted
him and he did not respond. We said: O Messenger of Allaah, we used to greet
you when you were praying and you would return the greeting. He said: “One
should concentrate fully in prayer.” Narrated by al-Bukhaari(1141) and
Muslim (538). 

It was narrated that Zayd ibn Arqam said: We used to speak
during the prayer; a man would speak to his neighbour whilst they were
praying, until the verse was revealed: “And stand before Allaah with
obedience [and do not speak to others during the Salaah (prayers)]”
[al-Baqarah 2:238]. Then we were commanded to remain silent and
forbidden to speak. 

Narrated by al-Bukhaari (1142) and Muslim (539). 

Al-Nawawi said: 

With regard to the words: “And stand before Allaah with
obedience [and do not speak to others during the Salaah (prayers)]”
[al-Baqarah 2:238], it was said: What is meant is obedient, or silent. 

The phrase “Then we were commanded to remain silent and
forbidden to speak” indicates that all kinds of human speech are forbidden
during prayer. The scholars are unanimously agreed that speaking
deliberately during the prayer, knowing that it is forbidden and for no
purpose invalidates the prayer. End quote. 

Sharh Muslim (5/27). 

Secondly: 

Al-Nawawi said in Adaab Qiraa’at al-Qur’aan: 

Chapter on the etiquette that is dictated by need. 

If he recites the words of Allaah (interpretation of the
meaning): “Allaah sends His Salaah (Graces, Honours, Blessings, Mercy) on
the Prophet (Muhammad), and also His angels (ask Allaah to bless and forgive
him)” [al-Ahzaab 33:56], it is mustahabb for him to say: “Sall-Allaahu
‘alayhi wa sallama tasleeman (May Allaah send blessing and  peace upon
him).” 

If one recites the verse “Is not Allaah the Best of
judges?” [al-Teen 95:5] or “Is not He (Allaah Who does that) Able to
give life to the dead?” [al-Qiyaamah 75:40], it is mustahabb to say: “Bala
wa ana ‘ala dhaalika min al-shaahideen (Yes indeed, and I am among those
who bear witness to that)”. If he recites “In what message after this
will they then believe?” [al-A’raaf 7:185] it is mustahabb to say “Aamantu
Billaah (I believe in Allaah).” 

All of that it is mustahabb for the reciter to say whether
during prayer or otherwise.  

A number of ahaadeeth have been narrated concerning that,
some of which are saheeh and some are da’eef. 

Abu Dawood (884) narrated that Moosa ibn Abi ‘Aa’ishah said:
A man was praying on the roof of his house, and when he recited: “Is not
He (Allaah Who does that) Able to give life to the dead?” [al-Qiyaamah
75:40] he said: “Subhaanaka fa bala (Glory to be You, yes
indeed).” They asked him about that and he said: “I heard it from the
Messenger of Allaah (peace and blessings of Allaah be upon him).”
Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

It was narrated from Ismaa’eel ibn Umayyah: I heard a Bedouin
say: I heard Abu Hurayrah say: The Messenger of Allaah (peace and
blessings of Allaah be upon him) said: ‘Whoever among you recites
Wa’l-teeni wa’l-zaytoon (“By the fig, and the olive” Soorat al-Teen 95),
when he reaches the end of it, “Is not Allaah the Best of judges?”
[al-Teen 95:5], let him say: “Bala wa ana ‘ala dhaalika min
al-shaahideen (Yes indeed, and I am among those who bear witness to
that)”.  Whoever recites La uqsimu bi yawm il-qiyaamah (“I swear
by the Day of Resurrection” Soorat al-Qiyaamah 75), when he reaches the
words “Is not He (Allaah Who does that) Able to give life to the dead?”
[al-Qiyaamah 75:40], let him say: “Bala (Yes indeed). And whoever
recites Wa’l-mursalaat (“By the winds (or angels or the Messengers
of Allaah) sent forth one after another” Soorat al-Mursalaat 77), when
he reaches the words “Then in what statement after this (the Qur’aan)
will they believe?” [al-Mursalaat 77:50], let him say: Aamanna Billaah
(We believe in Allaah).’ 

Narrated by Abu Dawood (887). It was classed as da’eef by
al-Albaani in Da’eef Abi Dawood. 

Shaykh Ibn ‘Uthaymeen said in al-Sharh al-Mumti’
(3/397-399): 

If the reciter says “Is not He (Allaah Who does that) Able
to give life to the dead?” [al-Qiyaamah 75:40], then he may say: Bala
(Yes indeed), or Subhaanaka fa bala (Glory be to You, yes indeed),
because there is a hadeeth about that from the Prophet (peace and
blessings of Allaah be upon him) and Imam Ahmad stated that. Imam Ahmad
said: If he recites “Is not He (Allaah Who does that) Able to give life
to the dead?” [al-Qiyaamah 75:40] during prayer or otherwise, he may
say, “Subhaanaka fa bala (Glory be to You, yes indeed)” both in
obligatory and naafil prayers. 

If he recites “Is not Allaah the Best of judges?” [al-Teen
95:5], he should say, “Subhaanaka fa bala (Glory be to You, yes
indeed)”.  

If he recites, “Is there any ilaah (god) with Allaah?”
[al-Naml 29:60], he may say, La ilaaha ill-Allaah (There is no
god but Allaah). End quote. 

Thus it is clear that these phrases are not included in the
speech that is forbidden during prayer and which invalidates it; rather what
is forbidden during prayer is normal human speech as the Prophet (peace
and blessings of Allaah be upon him) said: “This prayer is not the right
place for any of the people’s speech.” Narrated by Muslim (537). 

Al-Nawawi said: What this means is that it is not the right
place for any of the people’s usual speech and conversation. End quote. 

And Allaah knows best.