Muslim (279) narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The purification of the vessel of one of you, if a dog licks it, is to wash it seven times, the first time with soil.” 

And Muslim (280) narrated that ‘Abd-Allaah ibn al-Mughaffal said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “If a dog licks the vessel of one of you, let him wash it seven times and rub it with soil the eighth time.” 

In these two ahaadeeth the Prophet (peace and blessings of Allaah be upon him) explained how purification from the najaasah of a dog is to be done, which is by washing the vessel seven times, one of which should be with soil. Both are required. 

Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (1/73): 

There is no difference of scholarly opinion concerning the fact that the najaasah of a dog must be washed seven times, one of which should be with soil. This is the view of al-Shaafa’i. End quote. 

Al-Nawawi said in al-Majmoo’ (2/598): 

The scholars differed concerning the licking of a dog. Our view is that it makes whatever it licked naajis (impure), and the vessel must be washed seven times, one of which must be with soil. This is the view of the majority of scholars. Ibn al-Mundhir narrated the view that it is obligatory to wash it seven times from Abu Hurayrah, Ibn ‘Abbaas, ‘Urwah ibn al-Zubayr, Tawoos, ‘Amr ibn Dinar, Maalik, al-Awzaa’i, Ahmad, Ishaaq, Abu ‘Ubayd and Abu Thawr. Ibn al-Mundhir said: This is also my opinion. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: 

If the najaasah is somewhere other than on the ground and it is the najaasah of a dog, then it must be purified by washing it seven times, one of which must be with soil. End quote. 

Majmoo’ Fataawa Ibn ‘Uthaymeen (11/245). 

It is better if the first washing is the one that is done with soil, but if it is done otherwise that still achieves the purpose and purifies the place. 

Al-Nawawi said in al-Majmoo’ (2/598): 

It is mustahabb to use the soil in the first washing, and if that is not done then it is better to do it in any other time other than the seventh. But if it is done in the seventh it is permissible. It says in the reports in the Saheeh: “seven times” and “seven times, the first time with soil”; in one report it says “the last time” instead of “the first time.” In other reports it says, “seven times, the seventh with soil” and “seven times and the eighth time sprinkle it with soil.” Al-Bayhaqi and others narrated all these reports, which indicates that it is not essential that it be done the first time, rather that it be done one of those times. End quote. 

Washing with soil may be done in several ways: 

1-Washing with water then sprinkling soil on it

2-Sprinkling soil on it and following it with water

3-Mixing soil with water and washing the vessel with it.

This was stated by Shaykh Ibn ‘Uthaymeen in Sharh Buloogh al-Maraam, hadeeth no. 14.