Bukhari

Hadith Commentary:
It is established that such items found in the sea may be picked up, and then one should act according to whatever situation becomes apparent afterwards.
The fact that when the Israelite man split open the piece of wood he had found, he discovered his entrusted sum of money inside it, was the fruit of his good intention.
This should indeed be called the karamah (miraculous favor) of both of these righteous-hearted Israelites.
Otherwise, in ordinary circumstances, this matter is extremely delicate.
It is also established that some servants of Allah take the fulfillment of trusts (amanah) and the observance of covenants to such an extent—and such people are very rare.
Allamah Qastallani (rahimahullah) states:
“And the point of the chapter heading is his statement: ‘So he took it,’ and this is based on the principle that the law of those before us is also law for us, as long as nothing in our law comes that contradicts it—especially when it is reported in the form of praise for the one who acted upon it.” That is, here the point of the chapter heading is the narrator’s words:
“So he took it,” meaning he took it.
From this, the purpose of the chapter is established.
Because the law of those before us is also law for us,
as long as it does not contradict our law,
especially when the doer is praised in our law.
The Prophet (sallallahu alayhi wa sallam) praised both of these Israelites.
Their action, for this reason, became worthy of emulation for us.

Hadith Commentary:
(1)
From this it is understood that if something insignificant is found in a river or an open field, the one who picks it up may use it, and it is permissible for him to benefit from it. According to this hadith, the person picked up the piece of wood floating in the river to use as fuel. This is based on the principle that the previous Shari’ah is also a proof for us, provided it does not contradict our own Shari’ah. The Messenger of Allah (sallallahu alayhi wa sallam) narrated this incident as a matter of praise and commendation, and there is no report of him objecting to it.

(2)
There is no doubt that wood floating in a river or any insignificant item found in an open field may be picked up, but nowadays rivers are used for transporting goods. Trees are cut in the mountains and their wood is thrown into the rivers. Downstream, there are timber markets where merchants recognize and retrieve their goods from the river. It is not permissible to pick up such wood. Its nature is different. It cannot be considered a lost item, nor can it be regarded as insignificant.