It was narrated to us by Moḥammed bin Ibrāhīm bin ‘Isḥāq رحمه الله, who said – it was narrated to us by ʿAbd al-ʿAzīz ibn Yaḥya, who said – it was narrated to us by Moḥammed bin Zakariya, who said – it was narrated to us by Moḥammed bin ʿAmārah, from his father, from al-Ṣādiq Jaʿfar bin Moḥammed, from his father, from his forefathers عليهم السلام, who said:1

The Messenger of Allāh صلى الله عليه وآله said: A part of me2 shall be buried in the land of Khorasān. No believer will visit it except that Allāh will make Paradise obligatory for him, and his body will be made forbidden from the Fire.

1. al-Moḥsini states the chain is reliable, although raises questions about its contents [Mashraʿat Biḥār al-Anwār, Volume 2, Page 481]. We see no issues with its contents.
2. In reference to Imām al-Ridha عليه السلام.

حدثنا محمد بن إبراهيم بن إسحاق ( رحمه الله ) ، قال : حدثنا عبد العزيز ابن يحيى ، قال : حدثنا محمد بن زكريا ، قال : حدثنا محمد بن عمارة ، عن أبيه ، عن الصادق جعفر بن محمد ، عن أبيه ، عن آبائه ( عليهم السلام ) ، قال : قال رسول الله ( صلى الله عليه وآله ) : ستدفن بضعة مني بأرض خراسان ، لا يزورها مؤمن إلا أوجب الله عز وجل له الجنة ، وحرم جسده على النار .