Musnad Ahmad 307:21

'Abd al-Samad narrated to us, Harb ibn Thabit—who used to reside among Banu Sulaym—narrated to us, he said: Ishaq ibn 'Abd Allah ibn Abi Talhah narrated to us, from his father, from his grandfather, who said: A man recited [the Quran] in the presence of 'Umar, and he corrected him. The man said: "I recited it to the Messenger of Allah (may Allah bless him and grant him peace), and he did not correct me." He said: So we gathered at the Prophet (may Allah bless him and grant him peace). The man recited to the Prophet (may Allah bless him and grant him peace), and he said to him: "You have done well." He said: 'Umar felt something [of unease] because of that. The Prophet (may Allah bless him and grant him peace) said: "O 'Umar, the entire Quran is correct, as long as you do not make a [verse of] punishment into [a verse of] forgiveness, or [a verse of] forgiveness into [a verse of] punishment." 'Abd al-Samad said on another occasion: Abu Thabit, from his book.