حَدَّثَنِي مَالِكٌ، عَنْ إِسْحَاقَ بْنِ عَبْدِ اللَّهِ بْنِ أَبِي طَلْحَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ سَمِعْتُ عُمَرَ بْنَ الْخَطَّابِ، وَخَرَجْتُ، مَعَهُ حَتَّى دَخَلَ حَائِطًا فَسَمِعْتُهُ وَهُوَ، يَقُولُ وَبَيْنِي وَبَيْنَهُ جِدَارٌ - وَهُوَ فِي جَوْفِ الْحَائِطِ - عُمَرُ بْنُ الْخَطَّابِ أَمِيرُ الْمُؤْمِنِينَ بَخٍ بَخٍ وَاللَّهِ لَتَتَّقِيَنَّ اللَّهَ أَوْ لَيُعَذِّبَنَّكَ ‏.‏

Malik related to me from Ishaq ibn Abdullah ibn Abi Talha thatAnas ibn Malik said, "I heard Umar ibn al-Khattab, when I was goingout with him to visit an orchard, say to himself, and there was a wallbetween him and me and he was inside the garden, 'Umar ibn al-Khattab,amir al-muminin! Well done! Well done! By Allah, fear Allah or he willpunish you.' " Translation Not Available

Book reference: Muwatta Imam Malik Book 56 Hadith 1837
Web reference: Muwatta Imam Malik Book 56 Hadith 24