Abu Muawiya, whom I believe traced it back to the Prophet ﷺ, said, "Whoever visits a sick person and says, 'I ask Allah the Magnificent, Lord of the Mighty Throne, to cure you,' seven times, Allah will cure him, provided his appointed time has been deferred."Yazid narrated it to me without any doubt about its being traced back to the Prophet ﷺ, and he agreed with him on the chain of transmission.
حدثنا أبو معاوية حدثنا حجاج عن المنهال بن عمرو عن عبد الله بن الحارث
أبو معاوية أراه رفعه قال من عاد مريضا فقال أسأل الله العظيم رب العرش العظيم أن يشفيك سبع مرات شفاه الله إن كان قد أخر يعني في أجله وحدثني يزيد لم يشك في رفعه ووافقه على الإسناد
Its isnad is sahih. Abdullah bin Harith is the Ansari, the Basran, the relative by marriage of Ibn Sirin, and he is a trustworthy taabi. Abu Zura, Nasai, and others confirmed his trustworthiness. This hadith is a repetition of the one before it, meaning Minhal narrated it from two teachers from Ibn Abbas, namely Saeed bin Jubayr and Abdullah bin Harith. Then Ahmad narrated it right after, from Yazid bin Harun, from Hajjaj bin Artah, with his own isnad, without any doubt about it being attributed to the Prophet.