Imam Reza (as) Martydom and burial

 Imām Reza (as), peace be on him, summoned Harthema b. A‘yun in the darkness of the night. When Herthama came, the Imām said to him:
“O Herthama, this is the time of my departure to Allah, the Exalted, and of my joining my grandfather and my fore-fathers, peace be on them; the fixed term has come; this tyrannical one (i.e.  al-Ma’mūn) has decided to kill me with squeezed grapes and pomegranates. He will  have needles prodded into the grapes at the place of their storks; he will put some poison on the hand of one of his retainers and order him to knead pomegranates with it, that they may be stained with poison. He will summon me on the next day, bring the grapes and pomegranates near to me, and ask me to eat them. I will eat them, so the decision is valid and the decree is present.
“When I die, he (al-Ma’mūn) will say: ‘I will wash him with my hand.’ When he says that, be alone with him and say to him on my behalf: ‘He (al-Ridā) said to you: ‘Do not wash, shroud, and bury me. For if you do that, He (Allah) will hasten for you the punishment which He has delayed from you, and the painful thing of which you are cautious will befall you.’ So surely he will refrain (from doing that).”
The Imām added, saying: “When he (al-Ma’mūn) let you wash me, he will sit on one of his lofty houses in order to tower over the place where I will be washed. So, O Herthama, do not wash me until you see that a white tent is installed beside the house. When you see that, then carry me (while I am wearing) my garments in which I am, place me from behind the tent, and stop behind it; someone other than you will be with you; do not remove the tent from me lest you should see me and be destroyed.
“He (al-Ma’mūn) will tower over you and say to you: ‘O Herthama, do you not claim that none washes the Imām except an Imām like him? So who will wash Abū al-Hasan ‘Ali b. Mūsā (al-Ridā) while his son Mohammed is in Medina, in the country of al-Hijāz, and we are at Tūs?
“If he says that, say to him: ‘We say: Surely none must wash the Imām except an Imām like him. If an aggressor transgresses and washes the Imām, the Imāmate of the Imām will not be invalid because of the transgression of the one who washes him, nor will the Imāmate of the Imām after him be invalid because he is prevented from washing his father. If Abū al-Hasan ‘Ali b. Mūsā al-Ridā, peace be on him, had been allowed (to live) in Medina, his son Mohammed would have washed him apparent and uncovered. None will now wash him except a hidden one.’ When the tent is raised, you will see that I have been wrapped in my shrouds. Then put me into my coffin and carry me. When he (al-Ma’mūn) wants to dig my grave, he will make the grave of his father Hārūn al-Rashid as qibla for my grave, and that will never occur. When they strike the ground with the picks, they will dig nothing of it even the clipping of a finger-nail. When they do their best and it was difficult for them to do that, then say to them: ‘He (al-Ridā) has ordered me to strike one pick in the qibla of the grave of his (al-Ma’mūn’s) father, Hārūn al-Rashid.’ When you strike (with the pick), you will make it reach a dug grave and a standing shrine. When the grave is open, then do not lay me in it until the white water gushes out of the shrine wherein and it becomes full of water. When the water sinks, then lay me in that grave and bury me in that shrine ( Uyūn Akhbār al-Ridā, vol. 2, p. 247. Nūr al-Absār, p. 145.) 

Reference :The life of Imām'Ali Bin Mūsā al-Ridā