Hadithcore
Mishkat al-Masabih, 4363

Hilal b. ‘Amir quoted his father as saying he saw the Prophet (ﷺ) in Mina on a she-mule giving an address, wearing a red cloak, with ‘Ali in front of him repeating what he said. (This was because the crowd was too large for all to hear the Prophet. ‘AIi was repeating his words for the benefit of those too far away to bear the address) Abu Dawud transmitted it.

وَعَن هلالِ بن عَامر عَن أَبِيه قَالَ: رَأَيْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِمِنًى يَخْطُبُ عَلَى بَغْلَةٍ وَعَلَيْهِ بُرْدٌ أَحْمَرُ وَعَلِيٌّ أَمَامَهُ يُعَبِّرُ عَنْهُ. رَوَاهُ أَبُو دَاوُدَ

References1 variant
In-Book Reference
Book 22, Hadith 54