Arbaeen is a divine number whose use in verses of the Quran and Hadiths shows there are many secrets hidden in it.
Divine prophets and men of God have always honored the tradition of Arbaeen and left many legacies related to the concept of Arbaeen.
In fact, the word Arbaeen, which has to do with quantities, is used to refer to qualities in Islamic mysticism and the Hadiths about Arbaeen mostly draw attention to spiritual growth.
Most divine messengers, including Prophet Muhammad (PBUH), were appointed to prophethood at the age of 40. Maybe the reason is because one can embark on spiritual growth by the age of 40 but it becomes difficult after that.
The fourth use of Arbaeen in Quran (three others were mentioned earlier) is in Surah Al-Ahqaf and refers of the ground for man’s growth:
“We have charged the human to be kind to his parents. With much pain his mother bore him, and with much pain she gave birth to him; his bearing and weaning are thirty months. When he grows to manhood and attains his fortieth year, he says: ‘Make me so disposed My Lord, so that I give thanks for the favors with which You have blessed me, my father and mother, and that I will do good deeds that will please You. And, make me righteous and also my descendants. To You I repent, and I am among those who surrender.’” (Verse 15 of Surah Al-Ahqaf)
The phrase “grows to manhood and attains his fortieth year” shows that this age is the age of perfection of the intellect and highest capacity for growth.
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According to a prophetic Hadith, a person who devotes himself sincerely to God for forty days, streams of wisdom will flow from his heart to his tongue.
In other words, purifying one’s actions for God for 40 day will affect his heart and help him recognize esoteric teachings.