Visiting The Sick ,Dua For The Sick Person And Its Reward

1) Visiting The Sick

Laa Ba’sa Ṭahoorun inshaAllah
Don't worry, if Allah wills, it will be expiation (for your sins).

Narrated Ibn `Abbas:
Allah's Messenger (ﷺ) entered upon sick man to pay him a visit, and said to him, Laa Ba`Stuhoor inshaAllah “(Don't worry, Allah willing, (your sickness will be) an expiation for your sins)." 
Sahih al-Bukhari 5662

2) Dua For The Sick Person

As’alullaah-Al-‛Azeema Rabbal-‛Arshil-‛Azeemi An Yashfeek 
(recite seven times)
I ask Allah The Supreme, Lord of the Magnificent Throne to cure you.

Narrated Abdullah ibn Abbas:
The Prophet (ﷺ) said: If anyone visits a sick whose time (of death) has not come, and says with him seven times: As’alullaah-Al-‛Azeema Rabbal-‛Arshil-‛Azeemi An Yashfeek (I ask Allah, the Mighty, the Lord of the mighty Throne, to cure you, Allah will cure him from that disease).
Sunan Abi Dawud 3106

3) Reward of Visiting the Sick

Abu Rabi (ra) reported directly from Allah’s Messenger (ﷺ) as saying:
The one who visits the sick is in fact like one who is in the fruit garden of Paradise so long as he does not return.
Sahih Muslim 2568a

It was narrated that ‘Ali said:
“I heard the Messenger of Allah (ﷺ) say: ‘Whoever comes to his Muslim brother and visits him (when he is sick), he is walking among the harvest of Paradise until he sits down, and when he sits down he is covered with mercy. If it is morning, seventy thousand angels will send blessing upon him until evening, and if it is evening, seventy thousand angels will send blessing upon him until morning.’”
Sunan Ibn Majah 1442