Shama-il Tirmidhi Chapter 45, Hadith Number 2 (312).

Imaam Muhammad AI-Baaqir Radiyallahu ‘Anhu says that someone asked Ummul Mu’mineen ‘Aayeshah Radiyallahu ‘Anha: ‘How was the bed of Rasulullah Sallallahu ‘Alayhi Wasallam in your house?’ She replied: ‘It was made of leather, in which was filled the coir of the date palm’. Hafsah Radiyallahu ‘Anha was asked how was the bed of Rasulullah Sallallahu ‘Alayhi Wasallam in your house? She replied: ‘It was a canvas folded into two, which was spread for Rasulullah Sallallahu ‘Alayhi Wasallam to sleep on. On one night I thought if I folded it into four and spread it, it would become softer. I folded it and spread it that way. In the morning Rasulullah Sallallahu ‘Alayhi Wasallam asked: ‘What did you spread for me last night? I replied: ‘It was the same bed, I only folded it into four so that it may become softer’. Rasulullah Sallallahu ‘Alayhi Wasallam said: ‘Leave it in its original form. Its softness deprived me of my prayers (tahajjud) at night’.

It means that he could not wake up for tahajjud, or he may have awakened later than usual. One sleeps more soundly on a soft bed, and also for a longer period. If it is a rough and uneven bed, firstly, one will not sleep with ease; secondly, one will awaken quickly.