Abu Muamar reported,
We were with Ali 
when a funeral procession passed by, and some people stood up for it.Ali asked, "Who gave you this ruling?"They replied, "Abu Musa."He said, "The Messenger of Allah ﷺ only did that once, when he was imitating the People of the Book. When he was forbidden from doing so, he stopped."
Ahmad 1200
Sahihحدثنا عبد الرزاق أخبرنا سفيان عن ليث عن مجاهد
عن أبي معمر قال
كنا مع علي رضي الله عنه فمر به جنازة فقام لها ناس فقال علي رضي الله عنه من أفتاكم هذا فقالوا أبو موسى قال إنما فعل ذلك رسول الله ﷺ مرة فكان يتشبه بأهل الكتاب فلما نهي انتهى
No references are available.
Its isnad is sahih. Sufyan is Thawri. Layth is Ibn Abu Salim, who is trustworthy, though some criticized him regarding his memory. The truth is that he is like other narrators, whose reports that show clear error are set aside. Some went to excess in criticizing him, to the point that Waki said, "Sufyan would not mention Layth by name," yet here he has named him. Saji even said, "Abu Dawud would not include his hadith in the Sunan that he compiled," but Hafiz responded to this in Tahdhib, saying, "So he said, but his hadith is indeed established in the Sunan, though rarely." Bukhari gave him a biographical entry in Al-Kabir 4/1/246 without mentioning any criticism of him.Abu Muamar is Abdullah bin Sakhbarah Azdi Kufi, a trustworthy follower of the companions. The hadith will come at greater length than this, also by way of Layth, in the Musnad of Abu Musa Ashari, 4:413. See also 1467.