It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "When any Muslim servant visits a sick person and recites this supplication: «أَسْأَلُاللّٰهَالْعَظِيمَرَبَّالْعَرْشِالْعَظِيمِأَنْيَشْفِيَكَ» 'I ask Allah, the Lord of the Magnificent Throne, to grant you healing,' Allah Almighty grants him well-being, provided that his appointed time of death has not arrived."
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 2138
Hadith Gradingحکم دارالسلام:حديث صحيح، حجاج بن أرطاة مدلس وقد عنعن، لكنه متابع.