Sahih Bukhari
Invoking Allah for Rain (Istisqaa)
Volume: 002 / Book Number: 017 / Hadith Number: 149
Narrated 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."