When the Verse: 'It is those who believe and confuse not their belief with wrong (i.e., worshipping others besides Allah): (6:82) was revealed, it became very hard on the companions of the Prophet (ﷺ) and they said, "Who among us has not confused his belief with wrong (oppression)?" On that, Allah's Apostle said, "This is not meant (by the Verse). Don't you listen to Luqman's statement: 'Verily! Joining others in worship with Allah is a great wrong indeed.' (31:13)

The Prophet. said, "The biggest of the great sins are: To join others in worship with Allah, to be undutiful to one's parents, and to give a false witness." He repeated it thrice, or said, "....a false statement," and kept on repeating that warning till we wished he would stop saying it. (See Hadith No.7, Vol. 8)

A bedouin came to the Prophet (ﷺ) and said, "O Allah's Messenger (ﷺ)! What are the biggest sins?: The Prophet (ﷺ) said, "To join others in worship with Allah." The bedouin said, "What is next?" The Prophet (ﷺ) said, "To be undutiful to one's parents." The bedouin said "What is next?" The Prophet (ﷺ) said "To take an oath 'Al-Ghamus." The bedouin said, "What is an oath 'Al-Ghamus'?" The Prophet (ﷺ) said, "The false oath through which one deprives a Muslim of his property (unjustly).

A man said, "O Allah's Messenger (ﷺ)! Shall we be punished for what we did in the Prelslamic Period of ignorance?" The Prophet (ﷺ) said, "Whoever does good in Islam will not be punished for what he did in the Pre-lslamic Period of ignorance and whoever does evil in Islam will be punished for his former and later (bad deeds).

Some Zanadiqa (atheists) were brought to `Ali and he burnt them. The news of this event, reached Ibn `Abbas who said, "If I had been in his place, I would not have burnt them, as Allah's Messenger (ﷺ) forbade it, saying, 'Do not punish anybody with Allah's punishment (fire).' I would have killed them according to the statement of Allah's Messenger (ﷺ), 'Whoever changed his Islamic religion, then kill him.'"

Abu Musa said, "I came to the Prophet (ﷺ) along with two men (from the tribe) of Ash`ariyin, one on my right and the other on my left, while Allah's Messenger (ﷺ) was brushing his teeth (with a Siwak), and both men asked him for some employment. The Prophet (ﷺ) said, 'O Abu Musa (O `Abdullah bin Qais!).' I said, 'By Him Who sent you with the Truth, these two men did not tell me what was in their hearts and I did not feel (realize) that they were seeking employment.' As if I were looking now at his Siwak being drawn to a corner under his lips, and he said, 'We never (or, we do not) appoint for our affairs anyone who seeks to be employed. But O Abu Musa! (or `Abdullah bin Qais!) Go to Yemen.'" The Prophet then sent Mu`adh bin Jabal after him and when Mu`adh reached him, he spread out a cushion for him and requested him to get down (and sit on the cushion). Behold: There was a fettered man beside Abu Muisa. Mu`adh asked, "Who is this (man)?" Abu Muisa said, "He was a Jew and became a Muslim and then reverted back to Judaism." Then Abu Muisa requested Mu`adh to sit down but Mu`adh said, "I will not sit down till he has been killed. This is the judgment of Allah and His Apostle (for such cases) and repeated it thrice. Then Abu Musa ordered that the man be killed, and he was killed. Abu Musa added, "Then we discussed the night prayers and one of us said, 'I pray and sleep, and I hope that Allah will reward me for my sleep as well as for my prayers.'"

When the Prophet (ﷺ) died and Abu Bakr became his successor and some of the Arabs reverted to disbelief, `Umar said, "O Abu Bakr! How can you fight these people although Allah's Messenger (ﷺ) said, 'I have been ordered to fight the people till they say: 'None has the right to be worshipped but Allah, 'and whoever said, 'None has the right to be worshipped but Allah', Allah will save his property and his life from me, unless (he does something for which he receives legal punishment) justly, and his account will be with Allah?' "Abu Bakr said, "By Allah! I will fight whoever differentiates between prayers and Zakat as Zakat is the right to be taken from property (according to Allah's Orders). By Allah! If they refused to pay me even a kid they used to pay to Allah's Messenger (ﷺ), I would fight with them for withholding it." `Umar said, "By Allah: It was nothing, but I noticed that Allah opened Abu Bakr's chest towards the decision to fight, therefore I realized that his decision was right."

A Jew passed by Allah's Messenger (ﷺ) and said, "As-Samu 'Alaika." Allah's Messenger (ﷺ) said in reply, "We 'Alaika." Allah's Messenger (ﷺ) then said to his companions, "Do you know what he (the Jew) has said? He said, 'As-Samu 'Alaika.'" They said, "O Allah's Messenger (ﷺ)! Shall we kill him?" The Prophet, said, "No. When the people of the Book greet you, say: 'Wa 'Alaikum.'"

A group of Jews asked permission to visit the Prophet (and when they were admitted) they said, "As- Samu 'Alaika (Death be upon you)." I said (to them), "But death and the curse of Allah be upon you!" The Prophet (ﷺ) said, "O `Aisha! Allah is kind and lenient and likes that one should be kind and lenient in all matters." I said, "Haven't you heard what they said?" He said, "I said (to them), 'Wa 'Alaikum (and upon you).

Allah's Messenger (ﷺ) said, "When the Jews greet anyone of you they say: 'Sam'Alaika (death be upon you); so you should say; 'Wa 'Alaika (and upon you).'"