It is permissible to use just tissues to clean oneself after defecating

Firstly: it is obligatory for the believer to obey the command of Allah and the command of His Messenger (peace and blessings of Allah be upon him), even if that goes against his desires and choices. Allah says (interpretation of the meaning): 

“It is not for a believer, man or woman, when Allah and His Messenger have decreed a matter that they should have any option in their decision. And whoever disobeys Allah and His Messenger, he has indeed strayed into a plain error”

[al-Ahzaab 33:36] 

“The only saying of the faithful believers, when they are called to Allah (His Words, the Qur’aan) and His Messenger, to judge between them, is that they say: ‘We hear and we obey.’ And such are the successful (who will live forever in Paradise).

 And whosoever obeys Allah and His Messenger, fears Allah, and keeps his duty (to Him), such are the successful”

[al-Noor 24:51-52]

If the believer heeds the command of Allah and the command of His Messenger (peace and blessings of Allah be upon him), then he will be rewarded for any difficulty that he encounters because of that. Hence the Prophet (peace and blessings of Allah be upon him) said to ‘Aa’ishah: “Your reward will be commensurate with your spending and your effort.” (al-Bukhaari, 1787; Muslim, 1211). 

With regard to the ruling on this issue: 

It is not obligatory to use water in order to remove any impurity that is left after relieving oneself, rather it is sufficient to use pebbles or tissue or any other things that may remove impurity. The evidence for that is the report narrated by Ahmad (23627) from ‘Aa’ishah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: “When any one of you goes and defecates, let him take with him three stones and cleanse himself with them, and that will be sufficient for him.” Classed as saheeh by al-Albaani in al-Irwa’ (44).  Ibn Qudaamah (may Allah have mercy on him) narrated that there was consensus among the Sahaabah that it is permissible to use only stones and that it is not obligatory to use water. Al-Mughni, 1/208. 

Although it is permissible to use just stones or tissue, it is preferable to use water. The evidence for that is the report narrated by al-Bukhaari ((150) and Muslim (271) from Anas ibn Maalik, who said: “The Messenger of Allah (peace and blessings of Allah be upon him) used to enter the washroom and I and another young boy like me would bring a leather vessel of water, and he would clean himself (istinja’) with water.” 

Al-Tirmidhi (19) narrated that ‘Aa’ishah said to the women: “Tell your husbands to clean themselves with water, for I am to shy to tell them. The Messenger of Allah (peace and blessings of Allah be upon him) used to do that.” Classed as saheeh by al-Nawawi in al-Majmoo, 2/101. Al-Tirmidhi (may Allah have mercy on him) said following this hadeeth: “This is what should be done according to the scholars who preferred that istinja’ (cleaning oneself after relieving oneself) should be done with water, but if it is done with stones that is good enough in their view. But they regarded it as mustahabb to clean oneself with water and regarded that as preferable.” 

So if you use both tissues and water, that is the best; start with tissue then use water. 

See al-Majmoo’ by al-Nawawi, 2/200 

And Allah knows best. May Allah bless our Prophet Muhammad.