While staying at Illyrio's manse in Pentos, Tyrion Lannister goes for a walk in the gardens and notices a poisonous variety of mushrooms growing in a cracked paving tile. He takes seven of them and stuffs them carefully in his boot.
The seven mushrooms
Tyrion sits on a stone bench drinking a flagon strongwine. He drops the flagon when it is empty and it rolls across the yard. As he goes to fetch it he notices some mushrooms growing up from a cracked paving tile. The mushrooms are pale white, with speckles and red-ribbed undersides dark as blood. He snaps one off and sniffs it and thinks to himself delicious…and deadly. There are seven mushrooms and Tyrion wonders if the Seven are trying to tell him something. He picks them all, snatches a glove from down the washerwoman's line, wraps them carefully, and stuffs them down his pocket. The effort makes him dizzy, so afterward he crawls back to the bench, curls up, and shuts his eyes. While it is not specifically mentioned if Illyrio knows that Tyrion picked mushrooms, it is implied that he does. When Illyrio and Tyrion have dinner later, mushrooms are served, and Illyrio insists the cook made them specially for Tyrion. Tyion almost eats them, but changes his mind. After Illyrio declines the mushrooms, Tyrion insists that in Westeros, it is considered impolite to poison your guest at supper. Even knowing what the mushrooms are, Tyrion considers eating a mushroom, but that thought frightens him, so he refuses. Illyrio then eats a mushroom, revealing that they were not poisoned, and declaring that Tyrion needs to start trusting him.
A Dance With Dragons
While heading down the Rhoyne on the Shy Maid, Tyrion is in a bad mood and thinks to himself that he still has the mushrooms he had plucked from the ground of Illyrio's manse, and there are days when he is sorely tempted to slip them into Griff's supper. The trouble is that Griff scarcely seems to eat.
After being kidnapped and being held captive by Ser Jorah Mormont in Volantis, Tyrion’s thoughts again go to his boot, to the mushrooms in the toe. His captor had not searched him quite as thoroughly as he might have. At the time Tyrion is under the (wrong) impression that Ser Jorah means to deliver him to his sister. Thinking of the mushrooms he says to himself that they can be a form of escape for him, and that his sister Cersei will not have him alive.
Much later, after being sold on the auction block in Meereen, he, Penny, and Jorah Mormont are on their way to their new master Yezzan zo Qaggaz. Tyrion has hears about Daenerys Targaryen, but he thinks to himself that he has no faith in royal rescues. He contemplates that, if necessary, he could see to their deliverance himself, believing the mushrooms in his boot to be enough for both him and Penny.
Later, when Yezzan zo Qaggaz's hated overseer Nurse contracts the pale mare, none of the other slaves will go near him, so Tyrion has to attend to him. Nurse succumbs to the disease; however, Tyrion is implied to have hastened his end and be the ultimate cause of Nurse's death via poisoning. When Nurse falls ill Tyrion serves him a nice hot dogtail soup “with slivers of mushroom in the broth,"  implying that the mushrooms used in the soup are the same poisonous ones that Tyrion took from Illyrio Mopatis's mance. As he is feeding Nurse, Tyrion says to himself, drink it down Nursey, that shitwater squirting from your arse needs to be replaced. The last word spoken by Nurse is "No," while Tyrion's last words to Nurse are "A Lannister always pays his debts.".
It is unknown whether Tyrion still has mushrooms left.