Prophet Moses was born with stammer. But when on Mount Sinai he was commanded by Allah to go to the Pharaoh of his time and invite him to embrace Islam, Moses prayed to Allah to remove his stammer so that he could convey the divine message without any difficulty. Allah asked Moses to eat a certain herb which he did and his stammer ended. “…and remove the impediment from my tongue [20:27].
In his old age, the stammer retuned and Moses started to have difficulty speaking. He remembered the herb so he ate that herb but instead of getting cured, his stammer became worse. Moses prayed to God and said the herb did not work for him. God replied: “you were cured by this herb the first time you ate it because you did it for Me; but when you ate it for yourself your stammer got worse because the herb is nothing but poison.” Iman Fakhr al-Din Razi says even a poison becomes a cure when Allah’s name is taken on it and when it is done not for personal benefit but for Allah.
From Fakhr al-Din Razi, Tafseer Kabeer (Arabic), Volume 1, Page 173.