Imamah is the Covenant of Allah

بسم الله الواحد القهار

وَإِذِ ابْتَلَىٰ إِبْرَاهِيمَ رَبُّهُ بِكَلِمَاتٍ فَأَتَمَّهُنَّ ۖ قَالَ إِنِّي جَاعِلُكَ لِلنَّاسِ إِمَامًا ۖ قَالَ وَمِنْ ذُرِّيَّتِي ۖ قَالَ لَا يَنَالُ عَهْدِي الظَّالِمِينَ

And when his Lord tried Ibraheem with certain words, he fulfilled them. He said: Surely I will make you an Imam of men. Ibrahim said: And of my offspring? My covenant does not include the unjust, said He.

[2:124] 

Sheikh Al-Kulayni: 

Ali ibn Ibrahim has narrated from his father from ibn abu ‘Umayr from Sama‘a from abu ‘Abd Allah(عليه السلام) who has said the following:
“About the words of Allah, the Most Majestic, the Most Gracious, ‘. . . fulfill your covenant with Me. . . .,’ the Imam said that it is {the covenant to acknowledge Wilayah of Amirul-Muminin(عليه السلام)} . . . and I shall fulfill Mine . . .[2:40]’ {Admit you in paradise.}”


Source: Al-Kafi, Vol. 1 Pg. 272

Grading: Sahih(authentic)

The status of the narrators can be seen in Rijaal section:

1193: Ali Ibn Ibrahim Ibn Hashim Al-Qummi
53: Ibrahim Ibn Hashim Al-Qummi
1538: Mohammad Ibn Abi Umayr Al-Azdi
854: Sama'a Ibn Mehran