Imaam Ibn al-Qayyim رحمه الله said,
"Faith is two halves: a half that is patience and a half that is gratitude. More than one from the Salaf said, 'Patience is one half of faith.' 'Abdullaah Ibn Mas'ood رضي الله عنه said, 'Faith is two halves, a half that is patience and a half that is gratitude.' For this reason Allaah, the Sublime, combined between patience and gratitude in His saying:
"Indeed in that are signs for every patient and grateful (person)." (Soorah Ibraaheem, 14:5)."
Then Imaam Ibn al-Qayyim رحمه الله goes on to mention ten considerations (indicated by those verses) which show patience is one half of faith. And they are (summarised):
One: That faith (eemaan) is a name for speech, action and intention and returns to two parts: action (fi'l) and abandonment (tark). Action is working obedience to Allaah, and that is gratitude. And abandonment is patience in staying away from disobedience, and the religion consists of these two affairs, performance of a commanded action and abandonment of the prohibited.
Two: Faith is built upon two pillars: certainty (yaqeen) and patience (sabr), and they are the two pillars mentioned in His saying, the Most High:
"And We made from amongst them leaders who guide by Our command when they showed patience and had certainty in Our signs." (Sooratus-Sajdah, 32:24)
Three: That faith is speech and action. Speech is the speech of the heart and tongue, and action is the action of the heart and limbs... these are the four pillars of faith upon which it is built and they return back to knowledge to knowledge and action. Included within action is restraining the soul from what has been prohibited. Both of them (knowledge and action) cannot be attained except with patience.
Four: The soul has two strengths (impulses): the impulse of embarking and the impulse of restraining, and it constantly alternates between these two impulses. Hence it embarks upon what it loves and refrains from what it dislikes. All of the religion is either embarking (upon something) or refraining (from something). Embarking upon obedience and refraining from disobedience to Allaah. And both of them cannot be attained except with patience.
Five: All of the religion is (composed of) hope (raghbah) and fear (rahbah). Hence the believer is one who hopes and fears. He, the Most High, said:
"Verily, they used to hasten on to do good deeds, and they used to call on Us with hope and fear, and used to humble themselves before Us." (Sooratul-Anbiyaa', 21:90)
...So you will not find a believer except one who hopes and fears. And hope and fear do not stand except upon the trunk of patience. His fear carries him upon patience and his hope leads him to gratitude.
Six: Everything the servant encounters (and performs) in this life does not exit from being either what benefits him in this life and the next, or harms him in this life and the next, or benefits him in one of the two abodes and harms him in the other. And the noblest of these types is that he does what benefits him in the Hereafter and leaves what harms in the Hereafter. This is the reality of faith, for doing what benefits him is gratitude and abandoning what harms him is patience.
Seven: The servant is never removed from from one of these three: from a commandment he performs, a prohibition he abandons and a decree that befalls him. His obligation towards all three is patience and gratitude. Doing what is commanded is gratitude, abandoning what is prohibited and having patience upon the decrees is patience.
Eight: The servant has two callers, one which calls to the world, its lusts and pleasures and one which calls to the Hereafter and what He prepared therein of eternal bliss. Disobeying the caller to lust and and desire is patience and responding to the caller to Allaah and the abode of the Hereafter is gratitude.
Nine: The entire religion is centred around two foundations: determination ('azm) and firmness (thabaat), and they are both mentioned in the hadeeth related by Imaam Ahmad رحمه الله and Imaam an-Nasaa'ee رحمه الله from the Prophet صلى الله عليه وسلم, "O Allaah, I ask You for firmness in the command and determination in guidance." The foundation of gratitude is having sound determination and the foundation of patience is having strength in firmness.
Ten: The religion is founded upon two foundations: truth and patience. They are mentioned in His saying, the Most High,
"And who mutually enjoin truth and mutually enjoin patience." (Sooratul-'Asr, 103:3)
When the servant has been requested to act by the truth himself and also implement it amongs the people, and this is the reality of gratitude, then he would be unable to do it without patience. Hence patience is half of faith, and Allaah knows best.
[Imaam Ibn al-Qayyim رحمه الله: 'Uddatus-Saabireen, pp. 176-180. As quoted in the footnotes to the English translation of Qaa'idah fis-Sabr wash-Shukr (A Principle Concerning Patience & Gratitude) by Shakhul-Islaam Ibn Taymiyyah رحمه الله, Hikmah Publications]