Sunan an Nasai
1941
Sahih+2

It was narrated that 'Abdullah bin Al-Mughaffal said:"The Messenger of Allah said: 'Whoever follows a Janazah until it is finished, he will have two Qirats, and whoever goes back before it is finished, he will have one Qirat

SahihAbu Ghuddah
SahihAl-Albani