(9) باب وُجُوبِ الإِحْدَادِ فِي عِدَّةِ الْوَفَاةِ وَتَحْرِيمِهِ فِي غَيْرِ ذَلِكَ إِلاَّ ثَلاَثَةَ أَيَّامٍ
Zainab bint Umm Salama (Allah be pleased with her) reported on the authority of her mother that a woman lost her husband. (As her eyes were ailing) they (her kith and kin) entertained fear about her eyes, so they came to Allah's Apostle (ﷺ) and sought permission for the use of collyrium, whereupon Allah's Messenger (ﷺ) said:
One among you used to spend one year in a dungeon dressed in worst clothes. (And at the end of this period) she threw dung at the dog which happened to pass that way and then she came out (of her 'Idda). Can't she (wait) even for four months and ten days?
Reference: Sahih Muslim 1488 c
In-book reference: Book 18, Hadith 76
USC-MSA web (English) reference(deprecated numbering scheme): Book 9, Hadith 3541