Why is Sadaqah called Sadaqah?

Shaykh Uthaimin (May Allah have mercy on him) said:

‘Sadaqah is called sadaqah because it shows the sidq (sincerity) of one’s iman when he gives it…Because wealth is loved by man and he doesn’t leave it except for something which is even more beloved to him…

And what’s more beloved than wealth with respect to the one who gives sadaqah is the reward that he will be getting…

So him spending from what he loves from this world in the hopes of attaining what he loves in the hereafter is proof of the sidq (sincerity) of his iman.’

(Sharh bulugh al-muram: 9/496)