As Salaam Aalekum
Once when someone was telling me something, I thought in my mind that what he was saying was wrong. When he asked me what I was thinking, I said nothing in a tone which indicated that what I was thinking was something not good (without intention)
Does my action constitute kufr.
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Your thinking does not constitute kufr.
And Allah Ta’āla Knows Best
Student Darul Iftaa
New York, USA
Checked and Approved by,
Mufti Ebrahim Desai.
This answer was collected from Askimam.org, which is operated under the supervision of Mufti Ebrahim Desai from South Africa.