The Prophet ﷺ said, "Allah has placed the truth upon Umar's tongue and in his heart."
حدثنا عبد الملك بن عمرو حدثنا نافع بن أبي نعيم عن نافع
عن النبي ﷺ قال إن الله تعالى جعل الحق على لسان عمر وقلبه
Its chain of transmission is sahih. Nafi bin Abu Naeem is Nafi bin Abdul-Rahman bin Abu Naeem, the reciter of the people of Madinah, and one of the seven famous reciters. He is a leading authority in recitation, having taught people for a long period, more than seventy years, and the leadership of recitation in Madinah came to rest with him, with people turning to him for it. He was very dark-skinned, with a handsome face, of good character, and had a playful nature. He is trustworthy. Ibn Main deemed him trustworthy, Ibn Hibban mentioned him among al-Thiqat, and Nasai said, "There is no harm in him." Bukhari gave him an entry in al-Kabir 4/2/87 without mentioning any criticism of him, and neither Bukhari nor Nasai mentioned him among al-Duafa. However, Ahmad considered him weak, saying, "The Quran was taken from him, but he is of no account in hadith." We favor the view of those who deemed him trustworthy. He has a substantial entry in Tabaqat al-Qurra by Ibn Jazari, number 3718.This hadith was narrated by Tirmidhi 4:315 at length, through the route of Abu Amir Aqadi, from Kharija bin Abdullah Ansari, from Nafi, from Ibn Umar, and Tirmidhi said, "Hasan sahih gharib from this route." So Nafi bin Abu Naeem was not alone in narrating it from Nafi from Ibn Umar, rather Kharija bin Abdullah bin Sulayman Ansari corroborated him, and he too is trustworthy. Ibn Hibban mentioned him among al-Thiqat, Abu Hatim said, "A shaykh whose hadith is sound," and Ibn Adi said, "There is no harm in him or in his narrations, as far as I am concerned." Ahmad considered him weak, while Ibn Main said, "There is no harm in him." Bukhari gave him an entry in al-Kabir 2/1/187 and said, "He heard from Nafi and Yazid bin Ruman," and neither Bukhari nor Nasai mentioned him among al-Duafa. Tirmidhi, as you can see, graded his hadith sahih, so deeming him trustworthy is the correct and preferred view.