Shia muhadith Mawla Salih al-Mazandarani in “Sharh Usool al-Kafi” 7/273:

Translation:

“And it was narrated from author of “Kashf al-Ghumma”, that he said: I was informed by truthful, that Sayyed Radiutdin Ali ibn Tawus didn’t agree that al-Mamun poisoned Ali ibn Musa (peace be upon them), and didn’t believe in it”