Sunan an-Nasa'i, Volume 4, Book 27, Hadith 3532 (DarusSalam) ⓘ

It was narrated from Zainab bint Umm Salamah, that Umm Salamah and Umm Habibah said: "A woman came to the Prophet and said: 'My daughter's husband has died, and I am worried about her eyes. Can I apply kohl to her?' The Messenger of Allah said: 'One of you used to stay (in mourning) for a year. Rather (the mourning period is) four months and ten (days). And when that year had passed she would go out and fling a piece of dung behind her.'"

أَخْبَرَنِي إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا جَرِيرٌ، عَنْ يَحْيَى بْنِ سَعِيدِ بْنِ قَيْسِ بْنِ قَهْدٍ الأَنْصَارِيِّ، - وَجَدُّهُ قَدْ أَدْرَكَ النَّبِيَّ صلى الله عليه وسلم - عَنْ حُمَيْدِ بْنِ نَافِعٍ عَنْ زَيْنَبَ بِنْتِ أُمِّ سَلَمَةَ عَنْ أُمِّ سَلَمَةَ وَأُمِّ حَبِيبَةَ قَالَتَا جَاءَتِ امْرَأَةٌ إِلَى النَّبِيِّ صلى الله عليه وسلم فَقَالَتْ إِنَّ ابْنَتِي تُوُفِّيَ عَنْهَا زَوْجُهَا وَإِنِّي أَخَافُ عَلَى عَيْنِهَا أَفَأَكْحُلُهَا فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ قَدْ كَانَتْ إِحْدَاكُنَّ تَجْلِسُ حَوْلاً وَإِنَّمَا هِيَ أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا فَإِذَا كَانَ الْحَوْلُ خَرَجَتْ وَرَمَتْ وَرَاءَهَا بِبَعْرَةٍ ‏" ‏ ‏.‏