Hadith #3199 (Sunan Abi Dawud)
Book: Funerals (Kitab Al-Jana'iz)
Narrator(s): Abdul Warith bin Sa'id bin Dhakwan عبد الوارث بن سعيد بن ذكوان, Abdullah bin 'Amr, Abu Ma'mar al-Mad'aq عبد الله بن عمرو - أبو معمر المقعد, Ali bin Shmakh علي بن شماخ
Grade: Hasan

The Prophet (ﷺ) said: When you pray over the dead, make a sincere supplication for him.

Source: https://parallelquran.com/hadith/sunan-abi-dawud-3199/