In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Allaah [The Exalted] said:
وَمَثَلُ ٱلَّذِينَ يُنفِقُونَ أَمۡوَٲلَهُمُ ٱبۡتِغَآءَ مَرۡضَاتِ ٱللَّهِ وَتَثۡبِيتً۬ا مِّنۡ أَنفُسِهِمۡ كَمَثَلِ جَنَّةِۭ بِرَبۡوَةٍ أَصَابَهَا وَابِلٌ۬ فَـَٔاتَتۡ أُڪُلَهَا ضِعۡفَيۡنِ فَإِن لَّمۡ يُصِبۡہَا وَابِلٌ۬ فَطَلٌّ۬ۗ وَٱللَّهُ بِمَا تَعۡمَلُونَ بَصِيرٌ
And the likeness of those who spend their wealth seeking Allah’s Pleasure while they in their ownselves are sure and certain that Allah will reward them (for their spending in His Cause), is the likeness of a garden on a height; heavy rain falls on it and it doubles its yield of harvest. And if it does not receive heavy rain, light rain suffices it. And Allah is All-Seer of (knows well) what you do.
This parable is the basis of the spending that is based on sincerity and truthfulness, for indeed seeking the Pleasure of Allaah is sincerity itself, and being certain in oneself is truthfulness. Indeed, the spender is faced with two [corrupt] affairs and if he is saved from them, he will be similar to what has been mentioned in this Aayah. Firstly is seeking praise or commendation, or [spending due] to one of his worldly ambitions, and this is the state of affairs of most those who spend. The second affair is the weakness, swaying and doubt in his soul as to whether he should [spend] or not. So, [the first corrupt affair] prevents one from seeking the Pleasure of Allaah and the second prevents him from being certain about spending, because indeed being certain is that the soul is given the courage, strengthened and pushed to spend, and this is its truthfulness. As for seeking the Pleasure of Allaah, this is sincerity.
When it is the case that the basis of spending to seek the Pleasure of Allaah and being certain about it is likened to a garden- the garden with numerous trees and is situated on an elevated place, then indeed it is the garden that is more perfect than the garden that is situated at a very low place. That is because when a garden is elevated, [exposed] to the air, exposed to the sun when it is rising and after it has risen, as well as at the time of its setting, then its fruits become more mature, wholesome, better and more numerous, because fruits increase in their good quality and good health [when more exposed] to the air and sun, as opposed to fruits that are grown under the shade.
When it is the case that nothing is feared for a garden that is situated in an elevated place, except insufficient water, therefore Allaah said, [أَصَابَهَا وَابِلٌ۬ – heavy rain falls on it], and thus its fruits are brought forth and it produces the blessings [decreed by Allaah]. So, it brings forth its fruit twice as much as that of others, or twice as much due to the heavy rain. This is similar to the situation of those who are foremost in spending.
Then Allaah said: [فَإِن لَّمۡ يُصِبۡہَا وَابِلٌ۬ فَطَلٌّ۬ – And if it does not receive heavy rain, light rain suffices it]. This is inferior to that of the heavy rain; but it is enough for the garden because of the [suitability and fertility] of its hotbed and the good manner in which it was planted, therefore it is enough to bring forth its benefit by way of the light rain. This is similar to the case of the pious people- those upon the second level of spending and they are of different stations in the sight of Allaah.
The people [whose spending is likened to the garden that received the heavy rain] are of a higher station – those who spend their wealth day and night, in secret and openly, and they give precedence to others over themselves, even though they are in need themselves. As for the people whose [spending is likened to the garden that received] the light rain, they are those upon a level in the middle. The likeness of these two groups and their deeds is that of a garden upon an elevated place, and their spending – a lot- is likened to a heavy rain and light rain. And just as one of the two types of rains necessitates that it will make the fruits of the garden [good and healthy] etc…likewise is their spending- whether a lot or small, after it is was based on seeking the Pleasure of Allaah and being certain in oneself regarding that. It [i.e. their spending] is pure in the sight of Allaah – increases and doubles.
[An Excerpt from Tareequl Hijratayn. by Imaam Ibnul Qayyim (rahmahullaah). Pages 454-455. Slightly paraphrased]