If a person find buried treasure in the ground what should he do?
Question

                I heard that if a person finds wealth or treasure buried in the grounds of a house that he has bought, he has to give one fifth of it in charity. How correct is this?.
Praise be to Allah.
      Firstly: 

    If this wealth or treasure has markings on it which indicate 
    that it was buried during the Jaahiliyyah, i.e. before Islam, whether it was 
    the wealth of the Romans or the Persians or anyone else, then in this case 
    one fifth of it (khums) must be given in charity, because the Prophet 
    (blessings and peace of Allah be upon him) said: “On rikaaz (buried 
    treasure) the khums (one fifth) must be paid.”

    Narrated by al-Bukhaari, no. 1499. 

    Shaykh al-Islam [Ibn Taymiyah] said: They [the scholars] are 
    agreed that the khums must be paid on buried treasure, as is proven in the 
    saheeh hadeeth from the Prophet (blessings and peace of Allah be upon him). 
    The rikaaz (buried treasure) concerning which there is no doubt is that 
    which was buried during the Jaahiliyyah; it is treasure that is buried in 
    the ground.

    End quote from Majmoo‘ al-Fataawa, 29/376 

    The issue of buried treasure has been discussed previously in 
    the answer to question no. 83746 

    But if this treasure was buried by Muslims, then it comes 
    under the heading of lost property and it must be announced and its owners 
    sought for one whole year. If its owners cannot be found, then it becomes 
    the property of the one who found it, provided that he will return it to its 
    owner if he finds him after that. 

    It says in al-Mawsoo‘ah al-Fiqhiyyah (23/102):

    There is no difference of opinion among the fuqaha’ that 
    things buried by Muslims come under the heading of lost property.

    It is known [to belong to the Muslims] if it has Islamic 
    markings on it, or the name of the Prophet (blessings and peace of Allah be 
    upon him) or one of the Muslim caliphs or governors, or a verse of the 
    Qur'aan, and the like. 

    It says in al-Mughni: If it has some Islamic markings 
    on part of it and some non-Islamic markings on part of it, it is also lost 
    property, because what appears to be the case is that it came into the 
    possession of a Muslim and it is not known that it went out of the Muslims’ 
    possession, so it is like the case where the entire item has Islamic 
    markings throughout. 

    What seems to be the case is that this is not the view of the 
    Hanbalis alone; rather it is the view of the other fuqaha’ also, as is clear 
    from their comments on recognising buried treasure of the Jaahiliyyah. 

    Ibn al-Qayyim said: Anything that has no markings on it comes 
    under the heading of lost property, because it is most likely to belong to 
    Muslims. 

    End quote from al-Turuq al-Hukamiyyah, 1/291 

    For information on the rulings on lost property, please see 
    the answer to question no. 5049. 

    Secondly: 

    If treasure is found in a house or a land that one has bought 
    or been given as a gift, the scholars differed as to who owns the treasure. 

    The majority of fuqaha’ are of the view that it belongs to 
    the former owner or his heirs if they are alive, because the treasure was 
    placed in the ground, and when he took possession of it [the land] he took 
    possession of whatever was in it, and what was in the ground did not cease 
    to be owned by him when it was sold. 

    Imam Ahmad -- according to one report --, Abu Yoosuf and some 
    of the Maalikis were of the view that the buried treasure that is left after 
    paying the khums belongs to the later owner, because the treasure did not 
    come into the possession of the one who bought the land, because it was 
    deposited in it; rather it comes into the possession of the owner of the 
    land at the time when it is found. 

    End quote from al-Mawsoo‘ah al-Fiqhiyyah, 23/106 

    Ibn Qudaamah al-Maqdisi regarded the second view as correct, 
    then he said: Because buried treasure is not owned merely by buying the 
    house, because it is not part of the house; rather it was deposited in it. 
    Therefore it comes under the heading of things that are permissible for 
    anyone, such as grass, wood, and game that one may find found in the land of 
    others and take them, thus becoming more entitled to them.

    End quote from al-Mughni, 2/327 

    This is also the view favoured by Shaykh Ibn ‘Uthaymeen (may 
    Allah have mercy on him) when he said: With regard to this matter, it seems 
    that it belongs to the one who finds it. … and that is because it is 
    separate from the land, so it does not come under his ownership. Thus it 
    belongs to the one who finds it, just as if he found truffles or something 
    that comes out of the ground; it belongs to the one who finds it. 

    End quote from his comments on al-Kaafi, 3/108 

    And Allah knows best.