Using water for istinja is a matter of etiquette or recommended; because he (peace be upon him) did it once and left it another time, but it became after his era (peace be upon him) one of the Sunnahs by the consensus of the companions: like Taraweeh; as it was narrated by a group of companions including: Ali, Muawiya, Ibn Umar, and Hudhayfah ibn al-Yaman (may Allah be pleased with them): that they used to perform istinja with water after using stones, until Ibn Umar said: We did it and found it to be a remedy and purification, so washing with water is better; because the cleanliness achieved by it is more complete. And from Anas ibn Malik (may Allah be pleased with him): "The Messenger of Allah (peace be upon him) entered with a boy who had a small water container, and he placed it, then he went to a sidra tree, and the Messenger of Allah (peace be upon him) relieved himself and came out to us after having performed istinja with water," in Sahih Muslim 1: 227. And from Ali ibn Abi Talib (may Allah be pleased with him): "They used to use dung, and you are using stones, so follow the stones with water," in Sunan al-Bayhaqi al-Kubra 1: 106, and the Athar of Abu Yusuf 1: 7, and Musannaf Ibn Abi Shaybah 1: 142. And from Abu Huraira (may Allah be pleased with him), he said: The Prophet (peace be upon him) said: "This verse was revealed about the people of Quba (In it are men who love to purify themselves, and Allah loves those who purify themselves)" he said: They used to perform istinja with water, and this verse was revealed about them," in Sunan al-Tirmidhi 5: 280, and Sunan Abu Dawood 1: 58, and Sunan Ibn Majah 1: 128, but the addition "following the stones with water" was narrated by al-Bazzar from al-Zuhri from the hadith of Ibn Abbas with a weak chain as in the verification of the Ahadith of al-Ihya 1: 295, and see: al-Daraya 1: 95, and Nasb al-Rayah 1: 485. See: Al-Mabsut, 1/ 9-10, and Bada'i al-Sana'i, 1/ 21, and Al-'Inayah, 1/ 215, and Allah knows best.