The Quran

وَمِنهُم مَن يَستَمِعُ إِلَيكَ حَتّىٰ إِذا خَرَجوا مِن عِندِكَ قالوا لِلَّذينَ أوتُوا العِلمَ ماذا قالَ آنِفًا ۚ أُولٰئِكَ الَّذينَ طَبَعَ اللَّهُ عَلىٰ قُلوبِهِم وَاتَّبَعوا أَهواءَهُم

Muhammad Asad

Now among those [hapless sinners] are such as [pretend to] listen to thee, [O Muhammad,]¹ and then, as soon as they leave thy presence, speak [with scorn] unto those who have understood [thy message]:² "What is it that he has said just now?" It is such as these whose hearts God has sealed because they [always] followed but their own lusts³ -

Cf. 6:25 and 10:42-43.
Lit., "unto those who have been given knowledge", Sc., "of the truth" or "of thy message": i.e., the believers. The people spoken of in the above are the hypocrites among the contemporaries of the Prophet as well as all people, at all times, who pretend to approach the Qur'anic message with a show of "reverence" but are in their innermost unwilling to admit that there is any sense in it.
I.e., the "sealing" of their hearts (for an explanation of which see note 7 on 2:7) is a consequence of their "following but their own lusts".