In Islam Du`a’ is the essence of worship as stated in an authentic Hadith. Qadar (Divine predestination) is a pillar of faith in Allah, as the Prophet, peace and blessings be upon him, mentioned when asked by Angel Jibril (Gabriel), peace be upon him, about Iman (faith), saying: “It is to believe in Allah, His angels, His Books, His Messengers, and the Last Day, and to believe in Divine predestination, both the good and the evil thereof.” (Reported by Muslim)
The eminent Muslim scholar, Dr. `Atiyyah `A. Lashin, Professor of Jurisprudence at Al-Azhar University, states: “Qadar, or Divine predestination is of two kinds:
1. Unconditional predestination
2. Conditional predestination.
Du`a’ has nothing to do with the first kind, while it may change the second one. This ruling is based on the Hadith that reads: “Nothing could change the Qadar except Du`a’.”
(Musnad Ahmad, Vol. 5, Hadith no. 277; Sunan At-Tirmidhi, Vol. 3, Hadith no. 139; and classified as Hasan (good) Hadith by Al-Abaani in Sahih Al-Jami`, Hadith no. 7687)”
Shedding more light on this issue, Dr. Muzammil Siddiqi, former president of the Islamic Society of North America (ISNA) and member of the Fiqh Council of North America, answers:

“Du`a’ has great power; it can change many things. We should always pray to Almighty Allah and should never give up hope in the acceptance of Du`a’. Du`a’ is the most important act of worship. It does not mean that we or any other human or jinn have power, but it means that Allah, the Almighty, has power and He can do whatever He wills. Du`a’ is the recognition and acknowledgement of our dependence upon Allah, Most High, and our deep faith that He can change anything. There are many verses in the Qur’an and many Hadiths that emphasize the importance and necessity of Du`a’. The Prophet, peace and blessings be upon him, said, “Du`a’ benefits in the things that have happened as well as the things that have not happened yet. The calamity comes down, but the Du`a’ confronts it and they may continuously wrestle until the Day of Resurrection.” (Reported by Al-Hakim.)
In another Hadith, it is reported that the Prophet, peace and blessings be upon him, said, “Nothing can change the Qadar except the Du`a’…” This Hadith is reported by several authorities on Hadith, such as Imam Ahmad, Ibn Hibban, Al-Hakim, and Ibn Majah… etc. Most of the scholars of Hadith consider it an acceptable Hadith.
There is no contradiction between our faith in the power of Du`a’ to Allah and in our belief in the Qada’ and Qadar of Allah. Scholars have explained that there are two types of Qada’ (Allah’s Decrees):
a. those that are known as Qada’ Mubram (absolute decrees);
b. and those that are known as Qada’ Mu`allaq (conditional decrees).
The absolute decrees do not change, but the conditional one may change if and when the conditions are fulfilled. Such decrees are based on the fulfillment of causes, as well as Du`a’. However, it is important to keep in mind that every thing is in the pre-existent knowledge of Almighty Allah. The knowledge of Allah, Most High, does not change, but the conditional decrees may change. Allah Almighty says in the Qur’an: “Allah effaceth what He will, and establisheth (what He will), and with Him is the source of ordinance.” (Ar-Ra`d: 39)
Some people raise this question: if Allah Almighty knows the future and His knowledge is always true, then things must happen according to His knowledge. Then how can there be change and how can we say that we have freedom? Actually, Muslim scholars have pointed out that knowledge (`Ilm) is an illuminative, but not necessarily always an effective, attribute. Allah the Almighty does know everything, but in whatever areas He has given us freedom to act, we do act freely. We are responsible for the things in which we have freedom. He will judge us according to the freedom that He has given us.
The great scholar of Islam, Ibn Taymiyah, may Allah bless his soul, states: “Allah Almighty has created His creation with some causes. He has commanded His servants to follow those causes in order to receive His forgiveness, His mercy and His reward in this life and in the Hereafter. Whoever thinks that he or she can achieve anything simply by trust and by neglecting the causes that Allah Almighty has commanded him or her to follow; or whoever thinks that objects do not depend on the causes, is really mistaken.”