the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
Allah will not hold us accountable for our innermost thoughts. Rather, we will be judged according to how we act upon those thoughts.
Thinking ill of someone is not the same as speaking badly about them to other people.
The Prophet (peace be upon him) said: “Allah has pardoned my Ummah for what they speak in their minds as long as they do not act upon it or express it audibly. [Sahîh al-Bukhârî]
Source: Islam Today