The Book of Wills
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا مُحَمَّدُ بْنُ بِشْرٍ، حَدَّثَنَا هِشَامٌ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، أَنَّ رَجُلاً، أَتَى النَّبِيَّ صلى الله عليه وسلم فَقَالَ يَا رَسُولَ اللَّهِ إِنَّ أُمِّيَ افْتُلِتَتْ نَفْسُهَا وَلَمْ تُوصِ وَأَظُنُّهَا لَوْ تَكَلَّمَتْ تَصَدَّقَتْ أَفَلَهَا أَجْرٌ إِنْ تَصَدَّقْتُ عَنْهَا قَالَ " نَعَمْ "A'isha (Allah be pleased with her) reported that a man came to Allah's Apostle (ﷺ) and said:Allah's Messenger, my mother died all of a sudden without making any will. I think if (she could have the opportunity) to speak she would have made a Sadaqa. Would there be any reward for her if I give charity on her behalf? He (the Holy Prophet) said: Yes
From Sahih Muslim, whose reports the scholars accept as authentic.