Historical data of the four imams
Imam abu Hanifa: 80 a.H. - 150 a.H.
Imam Malik: 93 a.H. -179 a.H.
Imam a-Shafi'i: 150 a.H. -204 a.H.
Imam Ahmad ibn Hanbal: 164 a.H. - 241 a.H.
(See also Arabic Wikipedia)
Further the Caliph (king) al-Ma'moon ibn Haroon ar-Rasheed: 170 a.H. -218 a.H. (19th of Rajab). (See also Arabic Wikipedia)
When did the Mihnah start and end
Ibn Kathir stated in his al-Bidaya wa-(a)Nihaya البداية والنهاية (Volume 10):
In the following I'm translating text from Arabic language, as this translation is of my own take it carefully!
ثم دخلت سنة ثمان عشرة ومائتين
Then entered the year eighteen and two hundred (218 a.H.)
[ذكر أول المحنة والفتنة]
First mentioned misfortune (Mihna) and strife (Fitna)
في هذه السنة كتب المأمون إلى نائبه ببغداد إسحاق بن إبراهيم بن مصعب يأمره أن يمتحن القضاة والمحدثين بالقول بخلق القرآن.
In this year, al-Ma'mun wrote to his deputy in Baghdad, Ishaaq ibn Ibrahim bin Mus'ab, ordering him to test judges and hadith scholars by saying the creation of the Qur’an.
وأن يرسل إليه جماعة منهم، وكتب إليه يستحثه في كتاب مطول وكتب غيره قد سردها ابن جرير كلها، ومضمونها الاحتجاج على أن القرآن محدث وكل محدث مخلوق،
And to send him a group of them, and he wrote to him urging him in a lengthy letter and letters of others that Ibn Jarir listed all of them. Its content is the "proof" that the Qur’an is anew and every anew is created.
وهذا احتجاج لا يوافقه عليه كثير من المتكلمين فضلا عن المحدثين، فإن القائلين بأن الله تعالى تقوم به الأفعال الاختيارية لا يقولون بأن فعله تعالى القائم بذاته المقدسة، مخلوق، بل لم يكن مخلوقا، بل يقولون هو محدث وليس بمخلوق، بل هو كلام الله القائم بذاته المقدسة، وما كان قائما بذاته لا يكون مخلوقا،
Many of the scholars of Kalam do not agree with this "proof", as well as the hadith scholars, for those who say that Allah the Almighty performs voluntary actions do not say that the action of the Almighty, which exists in His holy self, is a creature, rather it was not a creature, rather they say it is updated and not a creature, rather it is the word of God the Exalted himself. Holy, and that which is self-standing shall not be a creature,
وقد قال الله تعالى ما يأتيهم من ذكر من ربهم محدث 21: 2 وقال تعالى ولقد خلقناكم ثم صورناكم ثم قلنا للملائكة اسجدوا لآدم 7: 11 فالأمر بالسجود صدر منه بعد خلق آدم، فالكلام القائم بالذات ليس مخلوقا، وهذا له موضع آخر. ...(Source)
Allah the Almighty says: "No mention comes to them anew from their Lord ..." (21:2), Allah the Almighty also says: "And We have certainly created you, [O Mankind], and given you [human] form. Then We said to the angels, "Prostrate to Adam"; ..." (7:11) So the command to prostrate was issued from him after Adam was created, because self-standing speech is not a creature, and this has another place. ...
The Mihnah survived al-Ma'amoon and his brother al-Mu'atassim bil-Laah (179 a.H. -227 a.H.) and al-Mu'tassims son Haroon al-Wathiq bil-Lah (200 a.H. -232 a.H.) who succeeded him in the reign. It ended in the reign of al-Wathiq's brother abu al-Fadl Ja'afar al-Mutawakkil (205 a.H. -247 a.H.). It is said that it duration was around 15 years.
As you see among the four scholars of now known madhhabs only imam Ahmad was alive.
- While a-Shafi'i's student al-Buwayti يوسف بن يحيى البويطي died in prison 231 a.H..
- Ibn Abd al-Hakam a hadith scholar whom's father and brothers were authorities in the maliki fiqh was an offspring of a rich family -in Egypt- a judge sentenced them to a fine of about one and a half million dinars, and many measures were taken against them to collect that fine, so their money and property were confiscated, and they were imprisoned, and the brother of Ibn Abd al-Hakam died in prison after being tortured.
- Ahmad ibn Nasr al-Khoza'iy أحمد بن نصر الخزاعي was executed 231 a.H. and his head was set in Baghdad while his hull was buried elsewhere and both where only buried together 237 a.H., it was reported that people heard his head reciting while he was put up.
A bit off-topic
The reign of al-Ma'moon was strange in many aspects he tried to reconcile with the past: For example he declared Musa al-Kadhim (the 8th shi'a imam) as his successor, then made up his mind, he somehow ended the oppression of the Omayyads and many internal fights. During his reign one could say an open minded and tolerant attitude made science blossom and bloom.
The oppression due to this "Mihnah" therefore sounds more and more strange, especially if we look at the facts: Not mu'tazilites (al-Mu'atazila) held this opinion, and not only them held it also the Jahamiyyah and some of the Ibadi's held it too. Those were doing the interrogation except with Ahmad ibn Abi Du'ad -whos influence increased with the years until he was seized and simissed by al-Mutawakkil- were not mu'tazilites, but also the Jahamy scholar Bishr al-Mursi بشر المريسي (died 218 a.H.) and the sunni scholar Yahya ibn Aktham were among them.
And there are many more strange factors in the whole case.
The position of the other 3 imams on this topic
Also note that the idea of the qur'an being a creation of Allah came up much earlier with the Jahamiyah and neither of the 3 remaining imams has been quoted to support this view: Most of the following statements were reported by imam Ahmad:
وقال النخعي: حدثنا أبو بكر المروذي قال: سمعت أبا عبد الله أحمد بن حنبل يقول: لم يصح عندنا أن أبا حنيفة كان يقول القرآن مخلوق. (Source)
Al-Nakha'i said: abu Bakr al-Marwadhi told us, he said: I heard abu Abdullah Ahmad ibn Hanbal saying: It is not true in our view that Abu Hanifa used to say the Qur’an is a creature.
وقال مالك رحمه الله تعالى: من قال القرآن مخلوق يوجع ضربا ويحبس حتى يتوب Malik, may Allah the Almighty have mercy on him, said: Whoever says the Qur’an is created has to suffer beating and imprisonment until he repents. (Source)
وقال الشافعي رحمه الله تعالى: في وصيته: القرآن كلام الله غير مخلوق.
Al-Shafi’i, may Allah the Almighty have mercy on him, said: In his will: The qur’an is the word of Allah, not created. (Source)
In case of abu Hanifa there are different views reported, but most scholars come to the conclusion that he changed his mind and held the view of ahl as-Sunnah.