It is narrated in Sahih Hadith that after burying the dead, it is Sunnah for those present to throw soil three times with both hands. Abu Hurairah (RA) said: "The Prophet (ﷺ) offered a janaza prayer. Then he went to the grave of the dead man and threw soil on him three times from the side of his head."

Reference: Sahih (Albani). Ibn Majah:1565