Ibn Qayyim al-Jawziyyah [d.751]
Source: Zaad al-Ma’aad (4/304-305)
translated for www.bakkah.net  [In the Name of Allaah, the Compassionate, the Most Merciful…]
In his excellent book, Zaad al-Ma’aad, in the section which includes the book, At-Tibb an-Nabawee, Shaykh al-Islaam Ibn al-Qayyim (may Allaah have Mercy on him) stated in his alphabetized discussion of medicines and nutrition:
– Prayer –
Allaah the Exalted has said:
( And seek help in patience and in prayer. Surely that is something quite difficult on (people) other than people of humility and submissiveness ) 
And He has said:
( O you who believe! Seek help in patience and in prayer! Verily, Allaah is with those who are patient. ) 
And He, the Exalted One, has said:
(And command your family to pray, and be patient upon that. We are not asking you for any provisions, (rather) We provide for you, and the favorable outcome is for (those with) taqwaa (consciousness of Allaah that produces righteous actions). ) 
And in the Books of Sunnah it is reported that the Messenger of Allaah (sallallaahu ‘alayhe wa sallam) used to hasten to prayer whenever an affair disturbed him. And previously discussed was the concept of healing most ailments through prayer before seeking out other ways of dealing with them.
– 1 – Prayer is something that causes one to receive sustenance.
– 2 – It is something that safeguards one’s physical health.
– 3 – It keeps away harmful things.
– 4 – It casts away illnesses.
– 5 – It strengthens the heart.
– 6 – It brightens one’s countenance.
– 7 – It delights the soul.
– 8 – It gets rid of laziness.
– 9 – It makes the limbs active.
– 10 – It increases one’s physical strength.
– 11 – It expands the chest (making one at ease and giving him insight).
– 12 – It is nourishment for the soul.
– 13 – It illuminates the heart.
– 14 – It safeguards one’s blessings.
– 15 – It repels catastrophes.
– 16 – It brings on blessings.
– 17 – It keeps away the Shaytaan (the Devil).
– 18 – It draws one close to Ar-Rahmaan (Allaah, the Most Merciful).
And overall it has an amazing effect on the health of the body and heart, and in strengthening them and expelling harmful pollutants from them. No two people have been afflicted with any disability, disease, or other calamity, except that the portion of the one who prays is less and his outcome is cleaner.
Also, prayer has an amazing effect on the evils of the dunyaa (the worldly life) and how it repels them, especially when the prayer is performed properly and completely, inwardly and outwardly. Nothing keeps the evils of the dunyaa at bay and brings on the benefits of it like prayer.
The reason behind this is that prayer is one’s connection to Allaah, the Mighty and Majestic. So based on the strength of a person’s relationship with his Lord, the Mighty and Majestic, the doors of goodness will accordingly be opened up for him, bad things and the reasons for them befalling him will be cut off, and the elements of success granted by His Lord, the Mighty and Majestic, will begin pouring in, along with safety and good health, wealth and worldly riches, relaxation, bliss, enjoyment, and all types of joyful affairs will be brought to him, and in an expedient manner, too.