There are many ahaadeeth which speak of its virtue, such as the words of the Prophet (peace and blessings of Allaah be upon him): “When the Muslim visits his (sick) Muslim brother, he is harvesting the fruits of Paradise until he returns.” Narrated by Muslim, 2568. 

The reward attained by the one who visits the sick is likened to the harvest reaped by one who gathers fruit.  

According to al-Tirmidhi (2008), the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever visits a sick person or visits a brother in Islam, a caller cries out to him: ‘May you be happy, may your walking be blessed, and may you occupy a dignified position in Paradise’.” Classed as hasan by al-Albaani in Saheeh al-Tirmidhi. 

Imam Ahmad narrated that Jaabir (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever visits a sick person is plunging into mercy until he sits down, and when he sits down he is submerged in it.” Classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 2504. 

Al-Tirmidhi (969) narrated that ‘Ali (may Allaah be pleased with him) said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “There is no Muslim who visits a (sick) Muslim early in the morning but seventy thousand angels send blessings upon him until evening comes, and if he visits him in the evening, seventy thousand angels send blessings upon him until morning comes, and he will have a garden in Paradise.” Classed as saheeh by al-Albaani in Saheeh al-Tirmidhi. 

Visiting the sick does not involve only those whom you know, rather it is prescribed for those whom you know and those whom you do not know. This was stated by al-Nawawi in Sharh Muslim.