Musannaf Ibn Abi Shayba 17:1198

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Ibn ‘Ajlan, from Zayd ibn Aslam, from his father, who said: ‘Umar entered upon Abu Bakr while he was holding his tongue, saying like this: "This is what has brought me to the sources [of destruction]."