Al-Kāfi - Volume 1 > The Path, Urged to be Maintained Steadfastly is Acknowledgement of the Divine Authority of Imam Ali (a.s.)
Hadith #1

1ـ أَحْمَدُ بْنُ مِهْرَانَ عَنْ عَبْدِ الْعَظِيمِ بْنِ عَبْدِ الله الْحَسَنِيِّ عَنْ مُوسَى بْنِ مُحَمَّدٍ عَنْ يُونُسَ بْنِ يَعْقُوبَ عَمَّنْ ذَكَرَهُ عَنْ ابي جعفر (عَلَيْهِ السَّلام) فِي قَوْلِهِ تَعَالَى وَأَنْ لَوِ اسْتَقامُوا عَلَى الطَّرِيقَةِ لاسْقَيْناهُمْ ماءً غَدَقاً قَالَ يَعْنِي لَوِ اسْتَقَامُوا عَلَى وَلايَةِ عَلِيِّ بْنِ أَبِي طَالِبٍ أَمِيرِ الْمُؤْمِنِينَ وَالاوْصِيَاءِ مِنْ وُلْدِهِ (عَلَيْهم السَّلام) وَقَبِلُوا طَاعَتَهُمْ فِي أَمْرِهِمْ وَنَهْيِهِمْ لاسْقَيْنَاهُمْ مَاءً غَدَقاً يَقُولُ لاشْرَبْنَا قُلُوبَهُمُ الايمَانَ وَالطَّرِيقَةُ هِيَ الايمَانُ بِوَلايَةِ عَلِيٍّ وَالاوْصِيَاءِ.

1. Ahmad ibn Mahran has narrated from ‘Abd al-‘Azim ibn ‘Abdallah al-Hassani from Musa ibn Muhammad from Yunus ibn Ya‘qub from the person whom he mentioned, from abu Ja‘far (a.s.). The Imam (a.s.) said the following about the words of Allah, the Most Holy, the Most High. “Had they (jinn and mankind) remained steadfast in their path (religion, Islam), We would certainly have given them abundant water to drink.” (72:16) It means, “Had people maintained steadfastness in respecting the Divine authority of Amir al-Mu’minin Ali ibn abu Talib (a.s.), and the Divine authority of his successors from his children and agreed to obey their orders and prohibitions He would have given them abundant water. He says, We would drench their hearts with faith. The path (al-Triqa) means to have faith in the Divine authority of Amir al-Mu’minin Ali (a.s.) and his successors from his children.”