It has been authentically reported from the Prophet (Sallallaahu alayhi wasallam) that he said:
(( ΕφΠσΗ ΟυΘφΫσ ΗαΕφεσΗΘυ έσήσΟϊ ΨσευΡσ ))
((Once a skin has been tanned. It is pure.)) [Muslim no: 336 and Abu Dawud no: 4123]
And he (Sallallaahu alayhi wasallam) said:
(( ΟφΘσΗΫυ ΜυαυζΟφ ΗαϊγσνϊΚσΙφ ΨυευζΡυεσΗ ))
((Tanning the skin of a dead animal purifies it.)) [Muslim no: 336 and Abu Dawud no: 4125]
Scholars have disagreed in this regard: Does this Hadith include all types of leather, or does it refer only to the skins of animals that have been made permissible through slaughter? There is no doubt that the skin of any dead animal which has been made permissible through slaughter, such as the camel, the cow, the sheep etc, is pure, and it is permissible to use it for everything, according to the most authorative opinion of the scholars.
As for the skin of the pig and he dog and other such animals, which are forbidden to slaughter, there is a difference of opinion regarding its purity amongst the scholars.
The best thing is to avoid using it, based upon the words of the Prophet (Sallallaahu alayhi wasallam):
((γσδ ΗΚψσήσμ ΗαΤψυΘυεσΗΚφ έσήσΟϊ ΗΣϊΚσΘϊΡσΓσ αφΟφνϊδφεφ ζΪφΡϊΦφεφ))
((Whoever avoids those matters which are unclear, has protected his religion and honour.)) [Al-Bukhari no. 52 and Muslim no. 1599]
And his words:
((ΟσΪϊ γσΗ νυΡφνϊΘυίσ Εφασμ γσΗ αΗσ νυΡφνϊΘυίσ))
((Leave whatever causes you to doubt, in favour of whatever does not cause you to doubt)) [At-Tirmithi no. 2518]