Why was Imam al-Reza (A.S.) Invited to Khurasan?

Burial Place of Imam al-Ridha (as)
We know that Harun al-Rashid fell sick in a trip toward the end of his life to Khurasan in order to suppress the popular rebellion there, and died after a little while and was buried in a garden belonging to Humaid b. Qahtaba.
When Imam al-Ridha (as) was martyred with the fatal poison that Ma’mun gave him, the latter had his Holiness buried next to his father, Harun. In the time, Humaid’s garden was located in a place called Sanabad, where after the martyrdom of Imam al-Ridha (as) was re-named Mashhad al-Ridha and later on was called simply Mashhad.
Since the time of burial (in 203/818.), his holy tomb turned into a place for manifestation of favors and bounties and divine blessings, as well as a pilgrimage place for the Shi‘ites and the devoted and faithful people. Gradually, it so happened that the grandeur and magnificence of the Holy Threshold overshadowed all other manifestations.
Works Relating Imam ‘Ali b. Musa al-Ridha (as)’s Traditions
As mentioned before, on many occasions Imam al-Ridha (as) held debates and arguments with the leaders of other religions such as Zoroastrians, Jews, Christians and even naturalists and materialists. Luckily, those debates and arguments are handed down to us in reliable volumes under the title, Al-Ihtijajat (Argumentations).
His precious traditions (ahadith), and wise sayings which were recorded in ‘Uyun Akhbar al-Ridha by Shaykh Saduq in the fourth century (A.H.),‘Ilal al-Sharayi‘, Tuhaf al-‘Uqul and other reliable books are at present available and illuminating the world of Islam and Shi‘ism.
Some Sayings of Imam al-Ridha (as):
I. Man’s friend is his intellect while ignorance is his enemy.
2. Expressing friendship to people is a half of intellect.
3. Those that are bestowed with a bounty must be open-handed and liberal to the members of their own family.
4. Imam al-Ridha (as) was asked about trust (in Allah); he answered such a trust is that you fear none but Allah.
5. Your helping out the weak is better than giving alms.
6. It is worthwhile that people provide the required facilities for their families and behave in such a way that the members of their families would not wish death for them.
7. Try to divide your night and day into four sections: one for worship and communion with Allah; one for earning livelihood; one for association with trustworthy friends who remind you of your deficiencies and who are sincere in their friendship; and one for resting and enjoying legitimate and healthy recreations, as desirable utilization of this section will enable you to carry out the other three sections.
8. Make utmost endeavor in satisfying the needs of the faithful, making them happy, and removing their distress; and know that apart from performing the religious obligations, nothing is better in the sight of the Exalted Allah than making the faithful people happy.
9. Do good to every good and bad person; if someone is worthy of that good, much the better, if he or she is not, it is you who are worthy of doing good.
10. No piety is more useful than to abstain from unlawful things and to avoid hurting the believers. (Durar al-Kalam)