It was narrated to us by Moḥammed bin Mūsa bin al-Mutawakkil رحمه الله, who said – it was narrated to us by ʿAlī bin Ibrāhīm, from his father, from Abī al-Ṣalt ʿAbd al-Salām bin Ṣāliḥ al-Harawi, who said:1

I heard al-Ridha عليه السلام say: By Allāh, there is none of us2 except that we are killed as martyrs.
He was asked: So who will kill you, oh son of the Messenger of Allāh?
He replied: The most evil of Allāh’s creation3 within my era will kill me with poison, then will bury me in a deserted house and abandoned land. Verily, whoever visits me in my estrangement, Allāh, Mighty and Majestic, writes for him the reward of one hundred thousand martyrs, and one hundred thousand truthful ones, and one hundred thousand ḥajjs and ʿumrahs, and one hungred thousand fighters in the way of Allāh. He will be resurrected within our ranks and will be put in the high levels of Paradise as our comrade.

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. There is discussions on what ‘us’ refers to, but the popular opinion is that it refers to all of the fourteen infallibles.
3. In reference to al-Ma’mūn.

حدثنا محمد بن موسى بن المتوكل ( رحمه الله ) ، قال : حدثنا علي بن إبراهيم ، عن أبيه ، عن أبي الصلت عبد السلام بن صالح الهروي ، قال : سمعت الرضا ( عليه السلام ) يقول : والله ما منا إلا مقتول شهيد . فقيل له : فمن يقتلك يا بن رسول الله ؟ قال : شر خلق الله في زماني ، يقتلني بالسم ، ثم يدفنني في دار مضيعة وبلاد غربة ، ألا فمن زارني في غربتي كتب الله عز وجل له أجر مائة ألف شهيد ومائة ألف صديق ومائة ألف حاج ومعتمر ومائة ألف مجاهد ، وحشر في زمرتنا ، وجعل في الدرجات العلى من الجنة رفيقنا .