Amro bin Shu'aib narrated on the authority of his father who reported on the authority of his grandfather, on the authority of Abdullah bin 'Amro(RAA) that the Messenger of Allah (ﷺ) said:
"One who becomes the guardian of an orphan, who owns property, must trade on his behalf and not leave it (saved and unused) until it is all eaten up by Zakah (which is paid yearly)." Related by At-Tirmidhi and Ad-Daraqutni with a weak chain of narrators.
English reference: Book 4, Hadith 632
Arabic reference: Book 4, Hadith 608