Mishkat al-Masabih 2671
Rafi‘ b. ‘Amr al-Muzani said:
I saw God’s messenger on a greyish she-mule addressing the people at Mina in the forenoon, while ‘Ali was repeating (He was repeating this for the benefit of those who were too far away to hear the Prophet) what he said, and some of the people were standing and others sitting. 

Abu Dawud transmitted it.
وَعَن رافعِ بنِ عمروٍ والمُزَني قَالَ: رَأَيْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَخْطُبُ النَّاسَ بِمِنًى حِينَ ارْتَفَعَ الضُّحَى عَلَى بَغْلَةٍ شَهْبَاءَ وَعَلِيٌّ يُعَبِّرُ عَنْهُ وَالنَّاسُ بَين قَائِم وقاعد. رَوَاهُ أَبُو دَاوُد