Sunan an-Nasa'i 1989

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنِ ابْنِ شِهَابٍ، عَنْ أَبِي أُمَامَةَ، أَنَّهُ قَالَ السُّنَّةُ فِي الصَّلاَةِ عَلَى الْجَنَازَةِ أَنْ يَقْرَأَ فِي التَّكْبِيرَةِ الأُولَى بِأُمِّ الْقُرْآنِ مُخَافَتَةً ثُمَّ يُكَبِّرَ ثَلاَثًا وَالتَّسْلِيمُ عِنْدَ الآخِرَةِ ‏.‏

(It was narrated that Abu Umamah said:"The Sunnah, when offering the funeral prayer, is to recite Umm Al-Qur'an) the Exxence of the Qur'an) quietly in the first Takbir, Then to say three (more) Takbir and to say the Taslim after the last one

Reference

Sunan an-Nasa'i 1989

In-book

Book 21, Hadith 172