ʿAbdullah b. Mas’ūd – Allah be pleased with him – said:

Singing sprouts hypocrisy (nifāq) in the heart as water sprouts greens and herbs.

Ibn Battah, Al-Ibānah Al-Kubrā Vol.2 p469, and Al-Bayhaqī, Al-Sunan Al-Kubrā Vol. 52 p231.

Ibn Qayyim Al-Jawzīyah – Allah have mercy on him – said in Madārij Al-Sālikīn, Vol.1 p487:

These are the words of someone who fully understood singing and its effects, for no one regularly sings or listens to song except that his heart falls into nifāq without him realizing. If such a person understood the reality of nifāq and its end he would see it in his own heart. Never do the love of song and the love of Qur`ān come together in a person’s heart except that one expels the other. I and others have witnessed how heavy the Qur`ān feels to singers and song-listeners; how they coil when it is recited and how they get angry with a reciter when he recites too long for them (in prayer etc); and how their hearts do not benefit from what he recites: they are not moved to do anything by it. But when the Qur`ān of Shaytān comes, lā ilāha illallāh! How they lower their voices and settle down! How their hearts feel at peace and how the crying and emotions start, how moved they are inwardly and outwardly and spend on clothing and perfume and staying up hoping for a long night ahead. If this is not nifāq then it is certainly the way to it and its foundation.