كتاب الوصية
25  
The Book of Wills
(2)
Chapter: The Reward For Charity Reaches The Deceased
(2)
باب وُصُولِ ثَوَابِ الصَّدَقَاتِ إِلَى الْمَيِّتِ ‏‏
Sahih Muslim 1004 c

A'isha (Allah be pleased with her) reported that a man said to Allah's Apostle (ﷺ):

My mother died all of a sudden, and I think if she (could have the opportunity) to speak she would have (made a will) regarding Sadaqa'. Will I be entitled to reward if I give charity on her behalf? He (the Holy Prophet) said: Yes.
حَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، أَخْبَرَنِي أَبِي، عَنْ عَائِشَةَ، أَنَّ رَجُلاً، قَالَ لِلنَّبِيِّ صلى الله عليه وسلم إِنَّ أُمِّيَ افْتُلِتَتْ نَفْسُهَا وَإِنِّي أَظُنُّهَا لَوْ تَكَلَّمَتْ تَصَدَّقَتْ فَلِيَ أَجْرٌ أَنْ أَتَصَدَّقَ عَنْهَا قَالَ ‏ "‏ نَعَمْ ‏"‏ ‏.‏
Reference : Sahih Muslim 1004c
In-book reference : Book 25, Hadith 17
USC-MSA web (English) reference : Book 13, Hadith 4002
  (deprecated numbering scheme)