Shama-il Tirmidhi Chapter 049, Hadith Number 004 (346).

Ibn 'Umar Radiyallahu 'Anhu reports: "Rasulullah Sallallahu 'Alayhi Wasallam called a hajjaam, who treated Rasulullah Sallallahu 'Alayhi Wasallam. Rasulullah Sallallahu 'Alayhi Wasallam inquired from him what tax or duty did he have to pay daily? He replied, three saa'. Rasulullah Sallallahu 'Alayhi Wasallam had it reduced to two saa', and gave him his remuneration".

Commentary
Apparently this is the same hadith about Abu Taybah Radiyallahu 'Anhu mentioned in the first hadith of this chapter. In the beginning he had to pay his master three saa', then Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam had it reduced to two saa'.