40

The Holy Book is clear that God considers 40 days a spiritually significant time period. Whenever God wanted to prepare someone for his purposes, he took 40 days. In addition to that, the number forty also plays a significant mark in human's life:

• Reading all content of Al Qur’an in 40 days is highly recommended by Prophet
Muhammad (Peace Be Upon Him) (Hadith Tirmidzi)
• Rasulullah PBUH received his prophethood at the age of 40
• Noah’s life was transformed by 40 days of rain.
• Moses was transformed by 40 days on Mount Sinai.
• David was transformed by Goliath’s 40-day challenge.
• Elijah was transformed when God gave him 40 days of strength from a single meal.
• Prophet Yunus was in a whales mouth for 40 days.
• Prophet Muhammad PBUH said, “In every one of you, all components of your creation
are gathered together in your mothers womb by 40 days” (Narrated in Saheeh Muslim)
• The average “Nifas” period (blood that is discharged due to childbirth) is 40 days.
• In the book of Numbers, the Israelites are shown to have first searched the
promised land (Canaan) for a duration of 40 days
• Primal Christians can also be recited to have held special regard for a time span
of 40 days. According to the synoptic Gospels, Jesus fasted and was "there in the
wilderness forty days, tempted of Satan... "
• Human body comprises around 40 trillion cells.
• Human red blood cell average life span is 40 days.

and offcourse, wise old saying said that "life begins at 40"[]