And Ali ibn al-Hasan narrated from Ali ibn Asbat, from Ala ibn Razin, from Muhammad ibn Muslim, who said:
Abu Abdillah عليه السلام, peace be upon him, said: “There is no seclusion (itikaf) except with fasting.”
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 4, p. 361