Taha 20:27

The third prayer وَاحْلُلْ عُقْدَةً مِّن لِّسَانِي يَفْقَهُوا قَوْلِي (And remove the knot from my tongue, that they may understand my speech - 20:27, 28). The story behind this "knot" is that as an infant Sayyidna Musa (علیہ السلام) lived with his own mother who used to suckle him and was paid for her services by the Pharaoh. When he was weaned, the Pharaoh and his wife 'Asiya ؓ adopted him, and took him away from his mother. One day Sayyidna Musa (علیہ السلام) caught hold of the Pharaoh's beard and slapped him on the face. Another version has it that he was playing with a stick and with it he struck the Pharaoh on his head. The Pharaoh was enraged and made up his mind to put him to death. His wife 'Asiya ؓ tried to pacify him and said, "0 King! Why do you take this matter so seriously? After all he is merely a child who does not understand things. You can test him if you like and you will find that he cannot distinguish between good things and bad things".

Thereupon, the Pharaoh ordered two trays to be brought. One was filled with live coal and the other with jewellery. It was expected that the child would be attracted by the brightness of the burning coal and reach for it because children are not normally drawn towards jewellery which is not as bright. This would have convinced the Pharaoh that what Sayyidna Musa (علیہ السلام) did was nothing more than the act of an innocent child. But Sayyidna Musa (علیہ السلام) was no ordinary child. Allah Ta'ala had chosen him to be a prophet whose instincts were unusual from the very moment of his birth. He put forth his hand to reach out for the jewellery instead of the coal, but Jibra'il (علیہ السلام) turned his hand away and placed it in the tray containing coal. He picked up a piece of burning coal and put it in his mouth and so burned his tongue. The Pharaoh was thus fully satisfied that the action of Sayyidna Musa (علیہ السلام) was not due to mischief but the result of a child's inability to distinguish between good and bad for himself. This incident caused an impediment in his speech which has been called عُقْدَةً (knot) in the Qur'an, and Sayyidna Musa (علیہ السلام) prayed to Allah Ta'ala to make loose this knot. (Mazharl and Qurtubi)

The first two prayers are of a general nature and sought Allah Ta` ala's help in all matters. The third prayer is for the removal of a disability because eloquence and fluency of expression are essential elements in the successful conduct of prophetic mission. In a subsequent verse Allah Ta'ala informed Sayyidna Musa (علیہ السلام) that all his prayers had been granted which would suggest that he was cured of his disability.

However, Sayyidna Musa علیہ السلام in his prayer to make Sayyidna Harun (علیہ السلام) his partner in the prophethood also said ھُوَاَفصَحُ مِنِّی لِساناً (He is more fluent in his tongue than me - 28:34) which would indicate that his speech's impediment was not fully cured and that the stammer persisted, though in a milder form. One of the defects which the Pharaoh found in Sayyidna Musa (علیہ السلام) was that وَلَا یَکَادُ یُبِینُ (he cannot express himself clearly - 43:52). Some people have argued that in his prayer Sayyidna Musa (علیہ السلام) had prayed to Allah Ta’ ala to loose the knot of his tongue only to the extent that others could understand his words. To that extent his stammer was cured but a trace of it still remained, which is not inconsistent with the grant of his prayer.