Hadith

Imam Reza (pbuh)

إنَّ لِكُلِّ إِمَامٍ عَهْداً فِي عُنُقِ أَوْلِيَائِهِ وَ شِيعَتِهِ وَ إِنَّ مِنْ تَمَامِ اَلْوَفَاءِ بِالْعَهْدِ وَ حُسْنِ اَلْأَدَاءِ زِيَارَةَ قُبُورِهِمْ فَمَنْ زَارَهُمْ رَغْبَةً فِي زِيَارَتِهِمْ وَ تَصْدِيقاً بِمَا رَغِبُوا فِيهِ كَانَ أَئِمَّتُهُمْ شُفَعَاءَهُمْ يَوْمَ اَلْقِيَامَةِ

Certainly, for every Imam there is a covenant incumbent on those who accepted their Wilaya and their followers. Without a doubt, that which completes and perfects the fulfillment of the covenant is visitation of their graves. Those who visit them longingly as a (practical) acknowledgment of what they long for (in their hearts), their Imams (pbut) shall certainly be their intercessors on the Day of Judgment

al-Kafi, v. 4, p. 567