What is the story behind Abu Darda's seeking forgiveness for forty Muslims, and how does it differ from seeking forgiveness for Muslim men and women in general?

What has been attributed to Abu al-Darda regarding his seeking forgiveness for forty Muslims is not established, just as the specification of seeking forgiveness for believing men and believing women twenty-seven times is mentioned in a weak hadith. Nevertheless, the encouragement to seek forgiveness for believing men and believing women is established, and one should do so frequently without specifying a certain number.