Shama-il Tirmidhi Chapter 021, Hadith Number 001 (123).

Jaabir bin Samura radiyallahu anhu reports, "I observed Rasoolullah sallallahu alaihe wasallam lean on a pillow which was on his left side".

Commentary.
It is permissible to put a pillow on both the right and left as stated in the hadith. The left is mentioned but only incidentally according to the rules and regulations of the muhadditeen (hadith scholars) the left side is not mentioned in the well -known narrations. Imam tirmizi has for this reason commented on this at the end of the chapter.