Sayyiduna 'Abdullah ibn 'Abbas (radiyallahu'anhuma) reports that Rasulullah (sallallahu'alayhi wasallam) passed by 2 graves whose inmates were being punished.
Rasulullah (sallallahu'alayhi wasallam) ordered that a branch of a date palm be broken in 2 pieces and placed on the graves, after which he said;
'Perhaps it will lighten the punishment for them (the 2 inmates) as long as these (branches) do not dry out'
(Sahih Bukhari, hadith: 216)
Because of the above, some Sahabah (radiyallahu'anhum) would bequest that after their burial, a date branch should be erected on their grave/placed in the grave. This has been reported from Sayyiduna Buraydah & Sayyiduna Abu Barzah (radiyallahu'anhuma)
(Sahih Bukhari, Chapter on (placing) a date branch on the grave, before hadith: 1361, Tarikh Dimashq, vol.62 pg.100 (Darul Fikr edition) & Sharhus Sudur, pg.405-406)
The grand commentator of Sahih Bukhari; Hafiz Ibn Hajar (rahimahullah) writes: "This (the hadith followed by the practice of the sahabi) is what is worth following.
(Fathul Bari, hadith: 216)
Also see: i'laus sunan, vol.8 pg.344 & Raddul Muhtar, vol.2 pg.245
