Hadith: Whoever visits a sick person whose time of death has not come and says, seven times "I ask Allah the Great, the Lord of the Great Throne, to cure you", except that Allah will cure him from that sickness

عن عبدالله بن عباس رضي الله عنهما مرفوعاً: «مَن عادَ مَريضا لم يحضُرهُ أجلُه، فقال عنده سَبعَ مراتٍ: أسأل الله العظيم، ربَّ العرشِ العظيم، أن يَشفيك، إلا عافاه الله من ذلك المرض».
[صحيح] - [رواه أبو داود والترمذي وأحمد]

‘Abdullāh ibn ‘Abbās (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: ''Whoever visits a sick person whose time of death has not come and says, seven times: ‘I ask Allah the Great, the Lord of the Great Throne, to cure you’, except that Allah will cure him from that sickness."
Sahih/Authentic. - [At-Tirmidhi]

Explanation

If someone visits a sick person whose time of death has not come and prays for him as specified in the Hadīth (I ask Allah the Great, the Lord of the Great Throne, to cure you), Allah will cure that person from his illness. If, however, the destined time of death has come, nothing will be useful, be it medicine or prayer. In this regard, Allah says: {And for every nation is a [specified] term. So when their time has come, they will not remain behind an hour, nor will they precede [it].} [Sūrat al-A‘rāf: 34]