Musnad Ahmad 30:2423

Yahya narrated to us, he said: 'Awf narrated to us, he said: Muhammad narrated to us, from Abu Hurairah, from the Prophet, may Allah bless him and grant him peace, who said: "Whoever follows the funeral procession of a Muslim, out of faith and seeking reward, and prays over it, and remains until it is buried, he returns with two qirats of reward, each qirat being like Uhud. And whoever prays over it and returns before it is buried, he returns with one qirat."