Al-Kāfi
Hadith 1321عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ هِشَامِ بْنِ سَالِمٍ عَنْ سُلَيْمَانَ بْنِ خَالِدٍ عَنْ أَبِي عَبْدِ اللَّهِ ( عليه السلام ) قَالَ الْخَلُّ يَشُدُّ الْعَقْلَ .
2. Ali ibn Ibrahim has narrated from his father from Ibn Abi ‘Umayr from Hisham ibn Salim from Sulayman ibn Khalid who has said the following: “Abu ‘Abd Allah (a.s.), has said, ‘Vinegar strengthens the power of understanding.’”