Hadith #5110 (Sunan Abi Dawud)
Book: General Behavior (Kitab Al-Adab)
Narrator(s): Abdullah ibn Shaddad, Dhar bin 'Abdullah al-Marhabi ذر بن عبد الله المرهبي, ibn Abbas, Jarir bin 'Abdul Hameed al-Razi جرير بن عبد الحميد أبو عبد الله الضبي, Mansur bin al-Ma'tamar al-Salmi منصور بن المعتمر بن عبد الله السلمي, Uthman bin Muhammad
Grade: Hasan

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.

Source: https://parallelquran.com/hadith/sunan-abi-dawud-5110/