Regarding the najasah of the dog and pig, the hadiths that have reached us can be divided into two categories; ones that contain the term ‘najis’ in them and others that don’t have the exact term but nonetheless necessitate their najasah. In these hadiths, the najasah of the dog and pig can be concluded judging from the questions that the imams have been asked regarding the leftovers of a dog and coming into contact with a pig and the corresponding answers they have given. In other words, the imam has been asked about praying in clothing that has come into contact with a pig, with the imam replying in the negative, which suggests the najasah of the pig.
Below, we will point out two hadiths as examples and for further information, we refer you to the hadith sources there are, but it is indispensable to keep in mind that understanding hadiths and deriving legal rulings from them call for expertise in different fields that the Islamic seminaries carry the burden of teaching. And now for the hadiths:
1- A hadith on the najasah of the dog: “أَنَّهُ سَأَلَ أَبَا عَبْدِ اللَّهِ (ع) عَنِ الْکَلْبِ فَقَالَ رِجْسٌ نِجْسٌ لَا یُتَوَضَّأْ بِفَضْلِهِ وَ اصْبُبْ ذَلِکَ الْمَاءَ وَ اغْسِلْهُ بِالتُّرَابِ أَوَّلَ مَرَّةٍ ثُمَّ بِالْمَاءِ”. Imam Sadiq (as) was asked about the dog. The imam answered: “It is najis. Throw away the leftover [water] of the dog and don’t perform wudu with it. First rub the container of the water with soil and then wash it with water.
2- A hadith on the najasah of the pig: “أَسْأَلُهُ عَنِ الثَّوْبِ یُصِیبُهُ الْخَمْرُ وَ لَحْمُ الْخِنْزِیرِ أَ یُصَلَّى فِیهِ أَمْ لَا فَإِنَّ أَصْحَابَنَا قَدِ اخْتَلَفُوا فِیهِ فَقَالَ بَعْضُهُمْ صَلِّ فِیهِ فَإِنَّ اللَّهَ إِنَّمَا حَرَّمَ شُرْبَهَا وَ قَالَ بَعْضُهُمْ لَا تُصَلِّ فِیهِ فَکَتَبَ (ع) لَا تُصَلِّ فِیهِ فَإِنَّهُ رِجْسٌ”. “I asked The imam (as) about the ruling of clothing that has come into contact with alcohol and pig meat; whether or not one can pray in it…the imam replied in the negative, saying not to pray in it because it is najis.”
 For further information, refer to the book Wasa’il al-Shia, vol. 3, chapter of the najasah of the dog, pp. 415-418 and the chapter of the najasah of the pig, pp. 418-419.