Musnad Ahmad 435:83

'Abd al-Rahman ibn Mahdi narrated to us, from Mu'awiyah ibn Salih, from Rabi'ah ibn Yazid, from 'Abdullah ibn 'Amir al-Yahsubi, who said: I heard Mu'awiyah narrating, and he was saying: "Beware of the hadiths of the Messenger of Allah (may Allah bless him and grant him peace), except for a hadith that was known during the time of 'Umar. For 'Umar (may Allah be pleased with him) was the most fearful of people regarding Allah, the Almighty and Majestic." I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Whoever Allah intends good for, He grants him understanding of the religion."