Volume 2, Book 17 “Dua’ for Rain (Istisqaa)“,
Number 149: Narrated by Ibn ‘Umar: Allah’s Apostle (p.b.u.h) said, “Keys of the unseen knowledge are five which nobody knows but Allah… nobody knows what will happen tomorrow; nobody knows what is in the womb; nobody knows what he will gain tomorrow; nobody knows at what place he will die; and nobody knows when it will rain.”
Explanation: It means without taking any external help. That is without using any machine or instrument or any kind of external help nobody knows what is in the womb and what is the rain forecast? Even with the help of machines one can never be one hundred percent sure about the predictions to turn out as one’s expectations.