Hawzah News Agency - Ali ibn Muhammad has narrated from Sahl ibn Ziyad from Muhammad ibn Sulayman from Ali ibn Ibrahim from ‘Abdallah ibn Sinan that Abu ‘Abdillah (al-Sadiq) 
said:
حُجَّةُ الله عَلَى الْعِبَادِ النَّبِيُّ وَالْحُجَّةُ فِيمَا بَيْنَ الْعِبَادِ وَبَيْنَ الله الْعَقْلُ.
“The hujjah (proof) of Allah over the worshippers is the Prophet ﷺ and the hujjah (proof) between Allah and the people is the intellect (al-'Aql).”
Al-Kāfi, v.1