`Abdullah bin `Amr bin Al-`as (May Allah be pleased with them) reported: Messenger of Allah (PBUH) said to me, "O Abdullah! Do not be like so-and-so. He used to get up at night for optional prayer but abandoned it later."[Al-Bukhari and Muslim].
The excellence of manners demands that if someone has something reprehensible in his character, the person concerned should not be mentioned but the shortcoming must be indicated so that people refrain from it. The second point that we learn from this Hadith is that if one starts a good deed, he should try his best to do it perpetually because its perpetuation is liked by Allah.