Abu Hurayrah narrated that the Messenger of Allah, sallallaahu ‘alayhi wa sallam, said:
“It is essential to feed the slave, clothe him and not burden him with work that he cannot handle.” An-Nawawi (a scholar) said: “The scholars have reached a unanimous consensus that it is impermissible to burden [a slave] with work that he cannot bear, and if he does, then he is obligated to either help him himself or with someone else.”
If any of his slaves became sick, he, sallallaahu ‘alayhi wa sallam, would visit them while they were sick, even if they were not Muslim:
Anas (a Companion) said:
“Once a Jewish boy who used to serve the Prophet of Allah, sallallaahu ‘alayhi wa sallam, fell sick. The Prophet of Allah, sallallaahu ‘alayhi wa sallam, went and paid him a visit. He sat by the boy’s head and said to him: ‘Accept Islam.’ The boy looked towards his father, who was also present, and the father said: ‘Obey Abu Al-Qaasim,’ and so the boy accepted Islam. When the Prophet of Allah, sallallaahu ‘alayhi wa sallam, left, he was saying: ‘All praise is due to Allah Who saved him from the Hellfire.’”
Thus, the Prophet of Allah, sallallaahu ‘alayhi wa sallam, was very keen on visiting his servant and calling him to Islam.