The Funeral Prayer (Salat Al-Janazah)
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 26
وَ رُوِيَ: «أَنَّ اَلْعِلَّةَ فِي ذَلِكَ أَنَّ اَللَّهَ تَعَالَى فَرَضَ عَلَى اَلنَّاسِ خَمْسَ صَلَوَاتٍُ فَجَعَلَ مِنْ كُلِّ صَلاَةٍ فَرِيضَةٍ لِلْمَيِّتِ تَكْبِيرَةً.
It has been narrated that the reason behind this practice is that Allah سبحانه وتعالى, the Exalted, has prescribed five obligatory prayers for people, and thus, one takbir is designated for the deceased from each of these obligatory prayers.
- Ṣaḥīḥ al-Faqīh