On the authority of the mother of the faithful, Aisha (may Allah be pleased with her), who said, "The Messenger of Allah (ﷺ) said, 'He who innovates something in this matter of ours (i.e. Islam) that is not of it will have it rejected (by Allah).'" [Bukhari & Muslim] In another version in Sahih Muslim, it reads, “He who does an act which we have not commanded, will have it rejected (by Allah).”

On the authority of Abdullah ibn Masood (may Allah be pleased with him) who said, "The Messenger of Allah (ﷺ), and he is the truthful, the believed, narrated to us, 'Verily the creation of each one of you is brought together in his mother’s womb for forty days in the form of a nutfah (a drop), then he becomes an alaqah (clot of blood) for a like period, then a mudghah (morsel of flesh) for a like period, then there is sent to him the angel who blows his soul into him and who is commanded with four matters: to write down his rizq (sustenance), his life span, his actions, and whether he will be happy or unhappy (i.e. whether or not he will enter Paradise). By the One, other than Whom there is no deity, verily one of you performs the actions of the people of Paradise until there is but an arms length between him and it, and that which has been written overtakes him, and so he acts with the actions of the people of the Hellfire and thus enters it; and verily one of you performs the actions of the people of the Hellfire, until there is but an arms length between him and it, and that which has been written overtakes him and so he acts with the actions of the people of Paradise and thus he enters it.'" [Bukhari & Muslim]

On the authority of Abdullah, the son of Umar ibn al-Khattab (may Allah be pleased with him), who said, "I heard the Messenger of Allah (ﷺ) say, 'Islam has been built on five [pillars]: Testifying that there is no deity worthy of worship except Allah and that Muhammad is the Messenger of Allah, establishing the salah (prayer), paying the zakat (obligatory charity), making the hajj (pilgrimage) to the House, and fasting in Ramadhan.'" [Bukhari & Muslim]

On the authority of `Umar (may Allah be please with him) who said, "While we were one day sitting with the Messenger of Allah (ﷺ) there appeared before us a man dressed in extremely white clothes and with very black hair. No traces of journeying were visible on him, and none of us knew him. He sat down close by the Prophet (ﷺ) rested his knees against the knees of the Prophet (ﷺ) and placed his palms over his thighs, and said, 'O Muhammad! Inform me about Islam.' The Messenger of Allah (ﷺ) replied, 'Islam is that you should testify that there is no deity worthy of worship except Allah and that Muhammad is His Messenger (ﷺ), that you should perform salah (ritual prayer), pay the zakah, fast during Ramadan, and perform Hajj (pilgrimage) to the House (the Ka`bah at Mecca), if you can find a way to it (or find the means for making the journey to it).' He said, 'You have spoken the truth.' We were astonished at his thus questioning him (ﷺ) and then telling him that he was right, but he went on to say, 'Inform me about Iman (faith).' He (the Prophet) answered, 'It is that you believe in Allah and His angels and His Books and His Messengers and in the Last Day, and in fate (qadar), both in its good and in its evil aspects.' He said, 'You have spoken the truth.' Then he (the man) said, 'Inform me about Ihsan.' He (the Prophet) answered, 'It is that you should serve Allah as though you could see Him, for though you cannot see Him yet He sees you.' He said, 'Inform me about the Hour.' He (the Prophet) said, 'About that the one questioned knows no more than the questioner.' So he said, 'Well, inform me about its signs.' He said, 'They are that the slave-girl will give birth to her mistress and that you will see the barefooted ones, the naked, the destitute, the herdsmen of the sheep (competing with each other) in raising lofty buildings.' Thereupon the man went off. I waited a while, and then he (the Prophet) said, 'O `Umar, do you know who that questioner was?' I replied, 'Allah and His Messenger know better.' He said, 'That was Jibril. He came to teach you your religion.'" (Muslim)

It is narrated on the authority of Amirul Mu'minin, Abu Hafs 'Umar bin al-Khattab (may Allah be pleased with him) who said, "I heard the Messenger of Allah (ﷺ) say, 'Actions are (judged) by motives (niyyah), so each man will have what he intended. Thus, he whose migration (hijrah) was to Allah and His Messenger, his migration is to Allah and His Messenger; but he whose migration was for some worldly thing he might gain, or for a wife he might marry, his migration is to that for which he migrated.'" (Bukhari & Muslim)