Sunan an-Nasa'i 114
It was narrated from 'Asim bin Laqit that his father said:
The Messenger of Allah (ﷺ) said: "When you perform Wudu', do so properly, and wash in between the fingers (Al-Asabi')."[2]

[1] Al-Asabi' is plural meaning fingers as well as toes, and the author mentioned only one narration on the topic whereas some of them clarify "of the hands and feet." So he mentioned the general wording amids chapters how to wash the feet.

[2] Part of this narration preceded under No. 87.
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنِي يَحْيَى بْنُ سُلَيْمٍ، عَنْ إِسْمَاعِيلَ بْنِ كَثِيرٍ، وَكَانَ، يُكْنَى أَبَا هَاشِمٍ ح وَأَنْبَأَنَا مُحَمَّدُ بْنُ رَافِعٍ، قَالَ حَدَّثَنَا يَحْيَى بْنُ آدَمَ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ أَبِي هَاشِمٍ، عَنْ عَاصِمِ بْنِ لَقِيطٍ، عَنْ أَبِيهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ إِذَا تَوَضَّأْتَ فَأَسْبِغِ الْوُضُوءَ وَخَلِّلْ بَيْنَ الأَصَابِعِ ‏"‏ ‏.‏