It depends on whether you are talking about your thinking right now, or whether you are talking about it on the past. Both sentences are grammatically correct.

Thought for past and think for present.

Hi, Your question has been well answered below - you are talking in the present tense, when you say "I think" in the first sentence, for something that has happened in the past. If you add 'now' e.g. "I 'now' think, he was sleeping when I came" this would be grammatically correct, as your 'thinking' is in the present even though your action of arriving is in the past. Clearly 'thought' in the second sentence is the past (participle) of the verb to think and correctly used here to describe what you were thinking at that time in the past. Hope this helps.

Hi! Without context, you could easily hear both of these sentences. I suppose that, depending on the narrative you are telling with these sentences, one might be preferred over the other.