It was better for you to say, when you entered your garden, `That which Allah wills! There is no power but with Allah!' If you see me less than you in wealth, and children.

Here he was urging and encouraging him to say that, as if he was saying, "When you entered your garden and looked at it and liked it, why wouldn't you praise Allah for the blessings He gave you and the wealth and children that He has given to you and not to others! Why did you not say `That which Allah wills! There is no power but with Allah!"'

One of the Salaf said,

"Whoever is delighted with something in his circumstances or his wealth or his children, let him say, `That which Allah wills! There is no power but with Allah!"'

This is based on this Ayah.

It was reported in the Sahih from Abu Musa that the Messenger of Allah said:

أَلَا أَدُلُّكَ عَلَى كَنْزٍ مِنْ كُنُوزِ الْجَنَّةِ 

Shall I not tell you about some of the treasure of Paradise? 

لَاا حَوْلَ وَلَاا قُوَّةَ إِلاَّ بِالله

There is no power or might but with Allah