It is well known that Abu Hanifa (radi Allahu 'anhu), did tahajjut every night. He would spend his night reciting the Quran. He had a neighbor who was an alcoholic, and he used to drink a lot and sing love poems. This used to bother the imam.
But one day, the imam did not hear this man's revelry, so he went and asked about him. They said, "Oh, so-and-so. They took him to jail." So, the very well respected imam went to the jail. He was the most respected imam and qaadi at the time in that place. When the ruler found out the imam went to the jail, he asked for the reason and was told that the imam was concerned about his neighbor who had been arrested. So, the ruler said to release the man, and he was released.
The neighbor then asked Abu Hanifa why he did that, and he replied, "Because you have a right upon me as a neighbor, and I have not been neglectful of that." That was the reason that the neighbor made tauba to Allah subhâna wa ta'âla [that is: embraced islam].