Sunan an Nasai
1653
Sahih Lighairihi+2

It was narrated from Al-Aswad, that Umm Salamah said:"The Messenger of Allah (ﷺ) did not pass away until most of his prayers were offered sitting down, except for the obligatory prayers

Sahih LighairihiAbu Ghuddah
SahihAl-Albani