In the name of Allah the Most Gracious the Most Merciful
The scholar is pardoned for his mistake, but not the one who knowingly follows that mistake!
By Shaykh Sulaymaan ar-Ruhayli May Allah preserve him
The shaykh mentions this benefit in his class “Dawratul ‘Idaad al-Mufti 2” in the second tape. He said,
If an Imaam says a statement (and is wrong) which is based on an opinion, because the evidence did not reach him, then he is excused and receives one reward . However, if someone comes after him and he knows the evidence which contradicts the Imaam’s statement, then it is not permissible for him to follow the Imaam. This is a very important issue, and it is necessary to pay attention to it.
 This can be understood from the Hadith of ‘Amr ibn al-Aws (may Allah be pleased with him) that the Messenger of Allah (peace be upon him) said,
“If a judge makes a ruling, striving to apply his reasoning (ijtihad) and he is correct, then he will have two rewards. If a judge makes a ruling, striving to apply his reasoning and he is mistaken, then he will have one reward.” [Muslim]
Translated by: Abu Awzaa’ee AbdusSalaam
الفقير الى الله