Authentic Supplication Number 155.

Abu Hurayrata  said that the Messenger of Allah  said: "Whoever attends the funeral procession till he offers the funeral prayer, for him there is [a reward equal to] one Qirat, and whoever accompanies it till burial, there is [a reward equal to] two Qirats." It was asked, "What are two Qirats?" He replied, "Like two huge mountains."

Reporters.
Reported in Saheeh Al-Bukhari #410 in the book of funerals in Volume 2 of Muhammad Muhsin Khan's translation.