Riyad as-Salihin

Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "Whoever follows the funeral procession and offers the funeral prayer for it, will get a reward equal to one Qirat, and whoever attends it till burial, will get a reward equal to two Qirat.'' It was asked, "What are two Qirat?'' He (ﷺ) replied, "Equal to two huge mountains."

[Al-Bukhari and Muslim].