Hadith 31-228

وَبِإِسْنادِهِ قالَ: قالَ النَّبِي‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ: عَلِيٌّ أَوَّلُ مَنِ اتَّبَعَني وَهُوَأَوَّل مَن يُصافِحَهُ الحَقِّ.

According to the same documentation, the Prophet (S) said, “Ali (a.s.) was the first man who followed me. He is the first one whom God will encounter (in the Hereafter.”