Ibn al-Qayyim (may Allaah have mercy on him) said: The madhhab of Abu Haneefah is the strictest in this regard, and his comments are among the harshest. His companions clearly stated that it is haraam to listen to all musical instruments such as the flute and the drum, even tapping a stick. They stated that it is a sin which implies that a person is a faasiq (rebellious evil doer) whose testimony should be rejected. They went further than that and said that listening to music is fisq (rebellion, evildoing) and enjoying it is kufr (disbelief). This is their words. They narrated in support of that a hadeeth which could not be attributed to the Prophet (peace and blessings of Allaah be upon him).
They said: he should try not to hear it if he passes by it or it is in his vicinity. Abu Yoosuf said, concerning a house from which could be heard the sound of musical instruments: Go in without their permission, because forbidding evil actions is obligatory, and if it were not allowed to enter without permission, people could not have fulfilled the obligatory duty (of enjoining what is good and forbidding what is evil).
(Ighaathat al-Lahfaan, 1/425).