Hadith 2:572
Sahih (Al-Albani)
(There is another chain) from Yahya bin Sa'eed that Al-Qasim bin Muhammad saw them sitting in Tashah-hud, so he mentioned the Hadith.
حَدَّثَنَا الْقَعْنَبِيُّ، عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّ الْقَاسِمَ بْنَ مُحَمَّدٍ، أَرَاهُمُ الْجُلُوسَ فِي التَّشَهُّدِ فَذَكَرَ الْحَدِيثَ
Original Reference: Sunan Abi Dawud 961