Imām Abū Jaʿfar Muḥammad ibn Jarīr al-Ṭabarī [d. 310 AH] said:

Abū Bakr al-Ṣiddīq [may Allāh be pleased with him] said: ‘Which earth will shelter me, and which sky shall shield me, if I deign to utter my own opinion concerning the Qurʾān-or [I say concerning it] that which I do not know.’

[Imām] Abū Jaʿfar explained: These narrations testify to the truthfulness of what we have postulated, that it is impermissible for anyone to voice their own opinion in interpretation of the aspects of the Qurʾān whose knowledge could only be acquired by the elucidation of the Messenger of Allāh [ﷺ], or that which has been [tacitly] alluded to by him.
 

Rather, the one who voices his opinion concerning the Qurʾān has made a grave error, even if his opinion happens to coincide with the truth. His conduct is erroneous as he has chosen to voice his own opinion.

Thus, his coinciding with the truth was not ascertained [by him when he spoke]; his opinion was but a coincidence, an estimation at best. The one who speaks concerning this dīn using guesses and estimations [from himself] has indeed speculated about Allāh with that which he has no knowledge concerning. This is something that Allāh the Exalted in Praise has made ḥarām on His servants in His Book. He says:

قُلْ إِنَّمَا حَرَّمَ رَبِّيَ الْفَوَاحِشَ مَا ظَهَرَ مِنْهَا وَمَا بَطَنَ وَالْإِثْمَ وَالْبَغْيَ بِغَيْرِ الْحَقِّ وَأَن تُشْرِكُوا بِاللَّهِ مَا لَمْ يُنَزِّلْ بِهِ سُلْطَانًا وَأَن تَقُولُوا عَلَى اللَّهِ مَا لَا تَعْلَمُونَ ‎﴿٣٣﴾‏

“Say, ‘My Lord has only forbidden immoralities – what is apparent of them and what is concealed – and sin, and oppression without right, and that you associate with Allah that for which He has not sent down authority, and that you say about Allah that which you do not know.'”
[Al-Aʿrāf, 7:33]

Thus, the one who speculates the interpretation of the Qurʾān—concerning that which could only be known by clarification from the Messenger of Allāh (ﷺ), for which Allāh has set him to clarify [for this ummah]—has indeed said about Allāh that which he does not know even if his speech coincides with the meaning intended by Allāh. For the one who speaks without knowledge, speculates about Allāh regarding matters he is ignorant of. This is the meaning of the ḥadīth narrated by Jundub that the Messenger of Allāh [ﷺ] said: ‘Whomever speaks about the Qurʾān with his own opinion and is correct, has made an error.’

He means that he has made an error in his action, by voicing his own opinion, even if it coincides with the unadulterated truth from Allāh. This is because his speculation was not that of one who is knowledgeable of the veracity of his opinion or its accuracy. He has, therefore, said about Allāh that which he does not know; he is sinful for his misconduct and perpetration of that which is forbidden and warned against.

Source: Tafsīr al-Ṭabarī 1:79
Translated by: Riyāḍ al-Kanadī