The Quran

أَلَم تَرَ إِلَى الَّذينَ نُهوا عَنِ النَّجوىٰ ثُمَّ يَعودونَ لِما نُهوا عَنهُ وَيَتَناجَونَ بِالإِثمِ وَالعُدوانِ وَمَعصِيَتِ الرَّسولِ وَإِذا جاءوكَ حَيَّوكَ بِما لَم يُحَيِّكَ بِهِ اللَّهُ وَيَقولونَ في أَنفُسِهِم لَولا يُعَذِّبُنَا اللَّهُ بِما نَقولُ ۚ حَسبُهُم جَهَنَّمُ يَصلَونَها ۖ فَبِئسَ المَصيرُ

Muhammad Asad

Art thou not aware of such as have been forbidden [to intrigue through] secret confabulations,¹ and yet [always] revert to that which they have been forbidden, and conspire with one another with a view to sinful doings, and aggressive conduct, and disobedience to the Apostle?² Now whenever such [people] approach thee, [O Muhammad,]³ they salute thee with a greeting which God has never countenanced;⁴ and they say to themselves, "Why does not God chastise us for what we are saying?"⁵ Hell shall be their allotted portion: they shall [indeed] enter it - and how vile a journey's end!

The prohibition referred to here arises from the Qur'anic statement, "No good comes, as a rule, out of secret confabulations - save those which are devoted to enjoining charity, or equitable dealings, or setting things to rights between people" (see 4:114 and the corresponding note 138). Although there is no doubt that, as the classical commentators point out, the "secret confabulations" spoken of in this passage relate to intrigues aimed against the Prophet and his followers by some of their unbelieving contemporaries, there is no doubt, either, that the passage has a general import, and is, therefore, valid for all times.
I.e., in the wider sense, disobedience to the Apostle's ethical teachings.
The reference to "approaching" the Prophet has here a twofold meaning, relating literally to his unbelieving contemporaries, and figuratively to an intellectual "approach" to his person and his teachings by hostile critics of all later times. The same observation is valid with regard to the next clause as well.
Lit., "with which God has never saluted thee". Historically, this is an allusion to the hostile attitude of the Jews of Medina towards the Prophet. It is recorded that instead of pronouncing the traditional greeting "Peace be upon thee" when encountering him, some of them used to mumble the word salam ("peace") in such a way as to make it indistinguishable from Sam ("death"); and they employed the same scurrilous play of words with regard to the Prophet's companions as well. (The relevant ahadith are quoted in full, with indication of the sources, by Tabari and Ibn Kathir in their commentaries on the above verse.) But see also the preceding note.
Sc., "if Muhammad is truly a prophet".