Sunan Abi Dawud 5110

Narrated Abdullah ibn Abbas: AbuZumayl said: I asked Ibn Abbas, saying: What is that I find in my breast? He asked: What is it? I replied: I swear by Allah, I cannot speak about it. He asked me: Is it something doubtful? and he laughed. He then said: No one could escape that, until Allah, the exalted, revealed: "If thou went in doubt as to what we have revealed unto thee, and ask those who have been reading the Book from before thee." He said: If you find something in your heart, say: He is the first and the Last, the Evident and the Immanent, and He has full knowledge of all things