MAN SUDDENLY GOT #800,000 ALERT IN HIS BANK ACCOUNT. ADVICE

It sounds silly but believe me or not it is PURELY real. I receive credit alert about a week now notifying me of 800,000 naira into my bank account from an unknown person, I was extremely surprised I even thought is 800 naira until I refresh my eyes and saw 800,000. 
I quickly login my mobile money app to see my balance and the money was there. 
I called the callable to if anyone sent money to me but non, though I didn't tell then about the 800,000 I asked tactically and uptil now nobody knows about the money. 
My fellow nairalanders please advise me before you people hear story that touch. 

Should I go and withdraw the money? 

Should I wait like months or even year? 

If it happens to be the bank mistake, will they hold me responsible after I spent the money? 

Please advise me on what to do. 
Thanks