It was narrated to us by Moḥammed bin al-Ḥassan رحمه الله, who said – it was narrated to us by Moḥammed bin al-Ḥassan al-Ṣaffār, from Aḥmed bin Moḥammed bin ʿĪsa, from Aḥmed bin Moḥammed bin Abī Naṣr al-Bazanṭi, who said:1

I read in the book of Abī al-Ḥassan al-Ridha عليه السلام: Tell my Shiʿa that visitation of me is equivalent with Allāh, Mighty and Majestic, to one thousand ḥajjs.
He (the narrator) said: I asked Abī Jaʿfar عليه السلام: One thousand ḥajjs?
He replied: Yes – by Allāh – one million ḥajjs for the one who visits him while having cognizance of his rights!

1. Al-Shaykh Hādi al-Najafi says this ḥadīth has an authentic chain [Mawsūʿat al-Aḥādīth ahl al-Bayt, Volume 4, Page 430]. Al-Moḥsini also states the chain is reliable, although states there needs to be much discussion regarding its contents [Mashraʿat Biḥār al-Anwār, Volume 2, Page 481]. We see no issues with its contents.

حدثنا محمد بن الحسن ( رحمه الله ) ، قال : حدثنا محمد بن الحسن الصفار ، عن أحمد بن محمد بن عيسى ، عن أحمد بن محمد بن أبي نصر البزنطي ، قال : قرأت في كتاب أبي الحسن الرضا ( عليه السلام ) : أبلغ شيعتي أن زيارتي تعدل عند الله عز وجل ألف حجة . قال : قلت لأبي جعفر ( عليه السلام ) : ألف حجة ؟ قال : إي والله ، وألف ألف حجة لمن زاره عارفا بحقه .