This was reported on the authority of 'Abdullah ibn Wahb one of imam Malik's most knowledgeable students from Egypt -see here in the Book "Al-Kifaya fi 'ilm ar-Riwaya الكفاية في علم الرواية"-:
أَخْبَرَنَا حَمْزَةُ بْنُ مُحَمَّدِ بْنِ طَاهِرٍ، ثنا الْوَلِيدُ بْنُ بَكْرٍ، ثنا أَبُو الْعَبَّاسِ تَمِيمُ بْنُ مُحَمَّدِ بْنِ تَمِيمٍ التَّمِيمِيُّ الزَّاهِدُ بِالْقَيْرَوَانِ، ثنا أَبُو الْغُصْنِ يَعِيشُ السُّوسِيُّ إِفْرِيقِيٌّ ثِقَةٌ قَالَ: سَمِعْتُ عَوْنَ بْنَ يُوسُفَ , مُغْرِبِيُّ ثِقَةٌ يَقُولُ: سَمِعْتُ عَبْدَ اللَّهِ بْنَ وَهْبٍ , يَقُولُ: §" كُنْتُ عِنْدَ مَالِكِ بْنِ أَنَسٍ جَالِسًا , فَجَاءَهُ رَجُلٌ قَدْ كَتَبَ الْمُوَطَّأَ يَحْمِلُهُ فِي كِسَائِهِ , فَقَالَ لَهُ: يَا أَبَا عَبْدِ اللَّهِ , هَذَا مُوَطَّؤُكَ قَدْ كَتَبْتُهُ وَقَابَلْتُهُ , فَأَجِزْهُ لِي , فَقَالَ: قَدْ فَعَلْتُ , قَالَ: فَكَيْفَ أَقُولُ أَخْبَرَنَا مَالِكٌ , أَوْ حَدَّثَنَا مَالِكٌ؟ قَالَ لَهُ مَالِكٌ: قُلْ أَيَّهُمَا شِئْتَ "
Here is my own attempt to translate it (including the narrator chain of al-Khateeb al-Baghdadi) -with additional information- take it carefully!:
We were informed by Hamza ibn Muhammd ibn Tahir, from al-Walid ibn Bakr, from abu al-'Abbas Tamim ibn Muhammad ibn Tamim at-Tamimi az-Zahid in al-Qyarawan, from Abu al-Ghusn Ya'eesh as-Susi -from Sousse- Ifriqi -from the former Roman province Africa- trustworthy said: I've heard 'Awn ibn Yusuf Mughraby trustworthy saying: I've heard 'Abdullah ibn Wahb saying:
"I was sitting at Malik bin Anas' auditorium, and a man came to him who had written (copied) al-Muwatta' carrying it in the fold of his clothes and he said to him, "Oh abu 'Abdullah, this is your Muwatta', I copied (made copy of) it and I've double-checked it, grant me your permission (Ijazah) to hand it down. He said: "I did", he asked, How can I express it "Malik informed us (akhbarana)"(literally: We were informed by Malik), or "Malik told us (haddathana)" (literally: We were told by Malik). Malik said to him, "Say whatever of both you want."
Again this is a report not a hadith by all means as it neither is a report of something the prophet () himself did, said or remained silent off (this is the strict meaning of hadith). Nor a report of the sahabah () nor tabi'yn () (extended meaning of hadith). Imam Malik is a tabi' at-Tabi'yn.