Hadith 31-237

وَبِإِسْنادِهِ قالَ: قالَ النَّبِي‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ: لا يَرى‏ عَوْرَتِي غَيْرِ عَلِيِّ إِلاّ كافِرٌ.

According to the same documentation, the Prophet (S) said, “Should anyone but Ali (a.s.) look at my private parts [76] he would be an atheist.”