Shaykh Saalih al-Fawzaan:
…With regard to matters relating to the Islaamic legislation and that of the religion, it is not permissible to speak about them based upon conjecture, because this will be a case of takalluf (unnecessary overburdening oneself). However, whoever has knowledge should respond and whoever does not have knowledge should say: ‘Allaah knows best.’ And whoever says: ‘I do not know,’ has given a response.
Imaam Maalik (rahimahullaah) was asked forty questions and he gave an answer to six of them. With regard to the rest, he said: ‘I do not know.’ So questioner said to him: ‘I have come from such-and-such place. I have traveled and tired out my riding animal and you say: ‘I do not know?’ So he said: ‘Ride your animal and go back to the land which you came from and say: ‘I asked Maalik and he said: ‘I do not know.”
This is not a defect that when a person does not know an answer regarding matters of the Islaamic legislation that he should say: ‘I do not know.’ Even if he is a scholar. The Messenger (sallallaahu`alayhi wa sallam) said:
«The one who is being asked about it does not know any better than the one who is asking.»(1)
He (sallallaahu`alayhi wa sallam) when he was asked certain questions and he did not possess revelation from Allaah, the Mighty and Majestic, he would wait until the revelation descended from Allaah, the Mighty and Majestic. Do you not recite [a number of times]: ‘They ask you about such-and-such. They ask you about such-and-such. Then say: ‘Such-and-such’?
«They ask you about intoxicants and gambling. Say: “In them is a great sin.”» [2:219]
«They ask you about the crescent moons. Say: “They are signs to mark time periods for the people and for the Hajj.”» [2:189]
So, the Messenger (sallallaahu`alayhi wa sallam) when he was asked a question and he did not have an answer, he would wait until revelation came down to him from Allaah. So that should be even more the case with anyone else besides him. He should wait until he asks someone else or until he researches the matter in the books of the people of knowledge to attain the answer. As for being hasty, then this is extremely dangerous and it contains showing bad manners towards Allaah, the Mighty and Majestic, since the one who gives the answer is answering about Allaah’s legislation. He is saying: ‘Allaah has made such-and-such permissible,’ or ‘He has made such-and-such forbidden,’ or ‘He has legislated such-and-such.’ So the matter is extremely dangerous.(2)
(1) The famous hadeeth of Jibreel (`alayhis-salaam).
(2) Sharh Usool-ith-Thalaathah, Shaykh Saalih al-Fawzaan. Translated by Aboo Talhah Dawud Burbank -rahimahullaah.