You need to cast the same spell 5 times in the same game. This is a little tricky since you can only have 4 of the same spell in a deck.
You will need a way to get the at least one of those cards up again from the graveyard (more if you're unlucky and doesn't draw all four from the deck).
There may be several ways to do this in this game, but here is how I did it :
I had a Charmbreaker Devils (4/4 Red creature with casting cost 1 Red and 5 colourless) in play. The card text is:
At the beginning of your upkeep, return an instant or sorcery card at random from your graveyeard to your hand. Whenever you cast an instant or sorcery spell, Charmbreaker Devil gets +4/+0 until end of turn.
As you can see, it will not take long until you can cast the same spell 5 times.
Another option is to have the creature Species Gorger in play. With it you will return a creature you control to its owner's hand (preferably, that should be you of course). Then you can recast the same creature again.
Species Gorger (6/6 Multicolour creature with casting cost 1 Blue, 1 Green and 3 colourless). The card text is:
At the beginning of your upkeep, return a creature you control to its owner's hand.
A third option is to activily use the black creature Gravedigger. It is specially useful if you have another Gravedigger (but can be used on another creature if you really want to) in your graveyard.
Gravedigger (2/2 Black creature with casting cost 1 Black and 3 colourless). The card text is:
When Gravedigger enters the battlefield, you may return target creature card from your graveyard to your hand.
A forth option is to use Dinrova Horror, where you can return a permanent to its owner's hand when the creature enters the battlefield. However I don't recommend using it since you will have to discard a card afterwards. The cards main purpose is to use it on a opponent. But this is still an option if you get lucky by not discarding the same card, or another card that you really need. So be aware if you try.
Dinrova Horror (4/4 Multicolour creature with casting cost 1 Black, 1 Blue and 4 colourless). The card text is:
When Dinrova Horror enters the battlefield, return target permanent to its owner's hand, then that player discards a card.
As mentioned in the comments you can also use spells to return creatures (or other permanents) to your hand. One of these is the blue Instant Voyage's End.
Voyage's End (Blue Instant with casting cotst 1 Blue and 1 colourless). The card text is:
Return target creature to its owners hand. Scry 1
Another one is Void Snar:
Void Snare (Blue Sorcery with casting cost 1 Blue). The card text is:
Return target nonland permanent to its owner's hand.
See comments for other tips. Thanks guys :)