The Obligation to mourn during the 'Iddah following the death of one's husband, but it is forbidden to mourn for more than three days in other cases
وَحَدَّثَنَاهُ أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ نُمَيْرٍ، ح وَحَدَّثَنَا عَمْرٌو النَّاقِدُ، حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ، كِلاَهُمَا عَنْ هِشَامٍ، بِهَذَا الإِسْنَادِ وَقَالاَ " عِنْدَ أَدْنَى طُهْرِهَا نُبْذَةً مِنْ قُسْطٍ وَأَظْفَارٍ " .
A hadith like this has been narrated on the authority of Hisham with the same chain of narrators but with a slight variation of words.