Allah said:
{Wa maa kaana lillahi an yattikhidah min walidin subhaanahu! Idhaa qadhaa amran fa innamaa yaqoolu lahu 'Kun' fa yakoon.}
{It not befitting that Allah should take a son - exalted is He! When He decides anything He merely says to it 'Be', and it is.} Maryam:35
{Tabaaraka alladhiy nazzala al-furqaana 'alaa 'abdihi li yakoona lil-'aalameena nadheeran. (1) Alladhiy lahu mulku as-samaawaati wa al-ardhi wa lam yattikhidh waladan wa lam yakun lahu shareekun fiy al-mulki wa khalaqa kulla shai'in fa qaddarahu taqdeeran.(2)}
{Blessed is He who sent down the criterion to His slave that he may be a warner to the worlds. (1) He to whom belongs all sovereignty in the heavens and the earth, who did not betake a son, who has no partner in his sovereignty and who created everything and established its proportions absolutely. (2)} Al-Furqaan:1-2
{Wa Allahu khalaqakum wa maa ta'maloon.}
{And Allah created you and that which you do.} As-Saffaat:96
The qadhaa' of Allah Most High is His eternal determination of the existence or non-existence of all things, actions and events.
The qadar of Allah is His eternal determination of the proportions and the amounts of all things and their condition at all points in time. No single atom can move (or not) in Allah's creation except by the qadar of Allah and His pre-existing will that same would happen.
Believe in Allah's predistination of all things is one of the six pillars of faith taught to us by the Prophet (sas) by means of his conversation with Jibreel in the famous hadith.
"...An tu'mina billahi wa malaa'ikatihi wa kutubihi wa rusulihi wa al-yaumi al-aakhiri wa tu'mina bi al-qadari - khairihi wa sharrihi."
"...To believe in Allah, His angels, His books, His Messengers and the Last Day. And to believe in predestination - of both good and bad." Muslim
Being a "pillar" means that it is a vital and indespensible part of faith. Without it, there is no valid faith. This is clear in the following advice of Ubada ibn As-Saamit to his son (which is also hadith, since it contains explicit narration from the Prophet (sas)):
"Ya bunayya innaka lan tajida ta'ma haqeeqati al-imaani hatta ta'lama anna maa asaabaka lam yakun li yukhti'aka wa maa akhta'aka lam yakun li yuseebaka fa inniy sami'tu rasoola Allahi yaqoolu: 'Inna awwala maa khalaqa Allahu al-qalam fa qaala lahu: uktub. Qaala: Yaa Rabbu maa aktub? Qaala: Uktub maqaadeera kulli shai'in hattaa yauma al-qiyaamati.' Yaa bunayya inniy sami'tu rasoola Allahi yaqoolu: 'Man maata 'alaa ghairi haadhaa ra laisa minniy."
"Dear son, you will surely never taste the sweetness of true faith until you know for certain that whatever comes to you was never going to miss you and whatever misses you was never going to come to you. For, verily, I heard Allah's Messenger (sas) say: 'The first thing which Allah created was the pen. Then He said to it: Write. It said: What shall I write, O Lord? He said: Write the proportions of all things until the day of Qiyama.' Dear son, I surely heard Allah's Messenger say: 'Whoever dies upon other than this is not of me.' Abu Daud
All of this is found in the Qur'an in many places. Failure to believe in Allah's qadar was one of the reasons for the nifaaq of the munaafiqeen ("hypocrites") in the time of the Prophet (sas) and after that about whom Allah said: "The hypocrites are in the deepest level of hell-fire." Belief in Allah's qadar is also what gave the believers strength in the face of the fitnah (trials) of the presence of munaafiqeen among the Muslims. Allah said, speaking about the hypocrites who used lame excuses to get out of jihad and then were gleeful if the Muslims suffered a defeat and envious if they achieved a victory:
{In tusibka hasanatun tasu'hum wa in tusibka museebatun yaqooloona: qad akhadhnaa amranaa min qablu wa yatawallau wa hum farihoon. (50) Qul: lan yuseebanaa illa maa kataba Allahu lanaa huwa maulaanaa wa 'alaa Allahi fal yatawakkal al-mu'minoon. (51)}
{If you encounter some good it irks them but if you encounter some calamity they say: We indeed took our preparations in time. And they turn away, overjoyed. (50) Say: Nothing will befall us except that which Allah has written for us. He is our Lord and upon Allah let all believers place their reliance.(51)} At-Tauba:50-51
So remember, dear brothers, that faith is not complete and you will surely never taste the sweetness of true faith until you realize this fact in everything you do and in all events which come to pass and in all decisions which you make in your life. Allah is there. Allah is closer to you than your jugular vein. Allah is above His throne above the seven heavens. Allah is the only one who can supply you whatever you need. Turn to Allah before turning to any others with your needs. Seek to please Allah in every way - in that which is easy for you and in that which is hard for you - so that Allah may be pleased with you and aid you in whatever you need. Allah is most Generous. And Allah is most Merciful. And Allah Most High OVERLOOKS MUCH. (Ya'foo 'an katheerin.).
{Wa asirroo qaulakum au ijharoo bihi, innahu 'aleemun bi dhaati as-sudoor. (13) Alaa ya'lamu man khalaqa wa huwa al-lateefu al-khabeer. (14)}
{And hide your statement or make it openly - He is surely knowledgeable of all that is in the hearts. (13) Does He not know who created? And He is the Subtle, the Knowing. (14)} Al-Mulk:13-14
{Maa asaaba min museebatin fiy al-ardhi wa laa fiy anfusikum illa fiy kitaabin min qabli an nabra'ahaa. Inna dhaalika 'alaa Allahi yaseerun.}
{No calamity occurs in the earth nor among yourselves but that it was in a book before it was brought to pass. That, surely, is easy for Allah.} Al-Hadeed:22