Imam Sadeq (pbuh), explaining this saying of Allah Almighty :{ We did indeed offer the Trust to the Heavens, the Earth and the Mountains, but they refused to undertake it, being afraid thereof. However, man undertook it; he was indeed unjust and foolish} said:" It's the Divine Leadership (Wilaya) of the Prince of the Believers (Ameer Almo'emineen) (pbuh)."
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الكافي ج1 ص413, بصائر الدرجات ص96, عنه البحار ج23 ص280, تأويل الآيات ج2 ص470, غاية المرام ج4 ص187, تفسير الثقلين ج4 ص312.