I saw Ali sacrificing two rams; so I asked him: What is this? He replied. The Messenger of Allah ﷺ enjoined upon me to sacrifice on his behalf, so that is what I am doing.  

رَأَيْتُ عَلِيًّا يُضَحِّي بِكَبْشَيْنِ فَقُلْتُ مَا هَذَا فَقَالَ إِنَّ رَسُولَ اللَّهِ ﷺ أَوْصَانِي أَنْ أُضَحِّيَ عَنْهُ فَأَنَا أُضَحِّي عَنْهُ