In Religion
Number Forty is a special number in the religious and Islamic culture.
In Mysticism
Muslim scholars, especially mystics, consider forty as a significant number in the fields of self-purification, finding one’s talents, and reaching moral heights.
In the Qur’an
The number forty has been mentioned with three different virtues in the Qur’an:
1. We have enjoined on man kindness to his parents: In pain did his mother bear him, and in pain did she give him birth. The carrying of the (child) to his weaning is (a period of) thirty months. At length, when he reaches the age of full strength and attains forty years, he says, "O my Lord! Grant me that I may be grateful for Thy favor which Thou has bestowed upon me, and upon both my parents, and that I may work righteousness such as Thou mayest approve; and be gracious to me in my issue. Truly have I turned to Thee and truly do I bow (to Thee) in Islam." (Al-Ahqaf: 15)
2. Allah said: "Therefore will the land be out of their reach for forty years: In distraction will they wander through the land: But sorrow thou not over these rebellious people. (Al-Ma’ida: 26)
3. “And remember We appointed forty nights for Moses, and in his absence ye took the calf (for worship), and ye did grievous wrong.” And “We appointed for Moses thirty nights, and completed (the period) with ten (more): thus was completed the term (of communion) with his Lord, forty nights. And Moses had charged his brother Aaron (before he went up): Act for me amongst my people: Do right, and follow not the way of those who do mischief.” (Al-Baqara: 51)
The Interpretations
There are different interpretations of the above mentioned verses concerning the number forty.
Verse 15 of Al-Ahqaf Surah could be related to the perfection of man at the age of forty. However, most interpreters believe that a man gains perfection at thirty three.
Imam Sadiq believed that man gains perfection at the age of thirty three and he will remain the same till he gets forty.
Anyway, many others believe that one’s mental ability is perfect at the age of forty, as Muhammad was appointed the prophet of God in the same age.
What is the Secret of Forty?
Some scholars like Sadr-al-Motalein Shirazi maintain that only Prophets and messengers of God know the secret of the number, forty.
There are some sayings indicating that God has made Adam out of soil in forty days. It took man forty days to turn from what it is to a divine creature. Therefore, in order for one to get back to his divine nature, one needs forty days of time.
source : tebyan