Muhammad ibn Ahmad ibn Yahya, from Mu‘awiyah ibn Hakim, from Mu‘ammar ibn Khallad:
(Imam) al-Ridha عليه السلام, peace be upon him, said: "There is no harm in praying among the graves as long as one does not take the grave as a Qiblah."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 244