6. Quran says that when a man attains maturity and reaches forty years he invokes a supplication. What is that supplication?

السلام عليكم ورحمة الله وبركاتة    

ﺑِﺴْـــــــﻢِﷲِﺍﻟﺮَّﺣْﻤَﻦِﺍلرَّﺣِﻴﻢ

Answer:

رَبِّ أَوْزِعْنِىٓ أَنْ أَشْكُرَ نِعْمَتَكَ ٱلَّتِىٓ أَنْعَمْتَ عَلَىَّ وَعَلَىٰ وَٰلِدَىَّ وَأَنْ أَعْمَلَ صَـٰلِحًۭا تَرْضَىٰهُ وَأَصْلِحْ لِى فِى ذُرِّيَّتِىٓ ۖ إِنِّى تُبْتُ إِلَيْكَ وَإِنِّى مِنَٱ لْمُسْلِمِينَ○

"My Lord! Grant me the power and ability that I may be grateful for Your Favour which You have bestowed upon me and upon my parents, and that I may do righteous good deeds, such as please You, and make my off-spring good. Truly, I have turned to You in repentance, and truly, I am one of the Muslims (submitting to Your Will)." [46:15]. 

Explanation:

It’s a prayer of gratitude, repentance and submission; invoking for doing the good deeds and for righteous offspring. 

At the age of forty, man's strength meets wisdom. As he grow older, then his strength reduces and wisdom takes over. At the age of forty, with the lessons taught by worldy life and by experiences of various kinds he will obtain spiritual maturity. He can distinguish between good and bad, truth and falsehood. He  turns to right guidance instead of deviation.

At this age a man will be most probably with the parents as well as with his offspring. He has to take the  responsibility of both.

 ■ He start realizing how Allah has guided and favoured so far his parents and himself .

 ■ He also realizes how he deviated from the straight path by the attractive worldy life and by the influence of Shaithan. 

 ■ So he wants to please Allah by doing good deeds as he comes to know by this time that evil deeds are erased by good deeds. So he likes to do many good deeds

■ He pray for the well being of his offspring.

■ His experiance in life teaches to repent for his sins. 

■ He comes to know that nothing reaches him except by the Will of Allah. 

■ So he fully submit himself to Allah.

Allah Subhanahuthala says in the next ayath:

"These are they from whom We accept the best of what they have done and pass over their evil deeds, among the dwellers of the garden; the promise of truth which they were promised" [46:16]

So from the age of forty a man has to be grateful to his parents and pray for them. Guide and pray for the well being of his offspring as well. Please his Creator by performing good deeds. Repent for his sins. Surrender fully to All Powerful.  If he continue to lead a good life till his end Allahuthala has promised Jannah for him.

But for some, the age of responsibility is “sufficiently long” as in the verse 35:37.  Despite a life-long chance, with continuous reminders from warners, the would be inhabitants of Hell have not learned a lesson. 

They will scream therein, "Our Lord, if you get us out of here, we will work righteousness, instead of the works we used to do." Did we not give you a life-long chance, with continuous reminders for those who would take heed? Did you not receive the warner? Therefore, taste (the consequences). The transgressors will have no one to help them". [35:37] 

 So  Allah says, "O you who believe, you shall repent to  Allah a firm repentance. Your Lord will then remit your sins and admit you into gardens with flowing streams. [66:8]

It is to be noted that Prophet Muhammad (SAW) attained prophet hood at this age. Heused to spend long periods of time in deep reflection in the Hira mountains and received his first revelation at the age of forty. Ibn Kathir said in his Tafsir of the Qur’an,  a man (or woman) attains forty years, his “complete intellect, understanding and patience reach the level of maturity.

So memorize this dua, supplicate and attain the pleasure of Allah and make a place in Jannah.

الحمدلله