Assalamualaikum w.b.t. Is ablution invalidated if a person eats or drinks? Can he then proceed to pray without first gargling or rinsing his mouth? Hope for an explanation.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
We begin with the statement of Allah SWT:
وَسَخَّرَ لَكُم مَّا فِي السَّمَاوَاتِ وَمَا فِي الْأَرْضِ جَمِيعًا مِّنْهُ ۚ إِنَّ فِي ذَٰلِكَ لَآيَاتٍ لِّقَوْمٍ يَتَفَكَّرُونَ
“And He has subjected to you whatever is in the heavens and whatever is on the earth – all from Him. Indeed, in that are signs for a people who give thought.” 
The above verse shows that Allah SWT has subjected everything in the heavens which includes the sun, moon and stars as well as and on earth from the animals (food and land travel), plants, ships and others for all of your benefit. All of these blessings are gifts from Allah SWT, He granted them to you as His blessings. Hence, you should worship only Him and never associate Him with anything. Indeed, everything that He has subjected for you is a sign and proof of His Oneness for those who contemplate His verses (signs of His greatness) as well as arguments and evidences for those who learn from them. 
Do eating and drinking invalidate ablution?
Regarding this matter, according to madhhab al-Syafi’I, eating doesn’t invalidate ablution, regardless of whether the food is cooked (with fire) or not, except camel’s meat. Scholars of madhhab al-Syafi’I differ on the opinion of camel’s meat (whether it invalidates ablution or otherwise) into two opinions. The jadid and famous opinion are that it doesn’t invalidate ablution and this is the final opinion according to scholars in madhhab al-Syafi’i. Whilst the second opinion – the Qadim opinion – camel’s meat invalidates ablution and this is a weak opinion according to scholars of madhhab al-Syafi’i. However, according to al-Nawawi, this weak opinion is stronger and more sahih in terms of its evidentiary proof and he is inclined towards this opinion in terms of its accuracy. 
This final opinion in madhhab al-Syafi’I is in line with a narration from Ibn Abbas R.Anhuma who said:
إِنَّما الوُضوءُ ممّا يخرجُ وليسَ مما يدخلُ
“Indeed, ablution is caused by what is excreted not what enters.” 
Furthermore, there is another hadith that clearly shows that the Prophet PBUH ate, then went to pray without reperforming his ablution or even rinsing his mouth. This is as stated in a narration from In Abbas R.Anhuma, he said:
تَعَرَّقَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كَتِفًا، ثُمَّ قَامَ فَصَلَّى وَلَمْ يَتَوَضَّأْ.
“The Prophet (ﷺ) ate of the meat of a shoulder (by cutting the meat with his teeth), and then got up and offered the prayer without performing the ablution anew.” 
The same applies to drinks. There is a narration from Anas bin Malik RA, where he said:
أنَّ رسولَ اللهِ ﷺ شَرِبَ لبَنًا فلم يمضمِضْ ولم يتوضَّأْ وصلّى
“The Messenger of Allah (ﷺ) drank some milk and he did not rinse his mouth nor did he perform ablution, and he offered the prayer.” 
Syaraf al-Haq al-‘Azim Abadi stated the above hadith shows that the act of rinsing one’s mouth after drinking milk or others that contain fat is not an obligation, rather it is a choice. 
Not only that, the act of rinsing one’s mouth after drinking milk or anything that contains fat is sunnah. This is as stated by Ibn Khuzaimah in his book, under the chapter which states the evidences of rinsing one’s mouth after drinking milk is sunnah to remove the fat (milk’s fat) in one’s mouth and not obligating rinsing one’s mouth after drinking it. 
Furthermore, al-Nawawi also stated that it is sunnah to rinse one’s mouth after drinking milk. Likewise, it is also sunnah to rinse one’s mouth after eating or drinking to avoid a person swallowing some leftover food in his mouth while he is praying. It is also to remove any stickiness and fattiness inside one’s mouth. 
According to the above arguments and discussions, we conclude it into several matters:
- The ablution of a person is not invalidated by the act of eating or drinking according to the majority of scholars. However, for those who eat camel’s meat, it is better to take a new ablution as a cautionary step and avoid the khilaf of those who obligate it.
- The same for a prayer of a person who eats and drinks then pray without first rinsing his mouth, it is valid.
- However, it is sunnah to rinse one’s mouth after eating and drinking especially before praying to remove any residual food or drink in between one’s teeth and if he swallows it during prayer, it will invalidate his prayer according to the famous opinion.
May Allah SWT gives us a clear understanding in religion. Amin.
 See al-Tafsir al-Muyassar, pg. 499.
 See al-Majmu‘ Syarh al-Muhazzab, 57/2.
 Narrated by al-Baihaqi (8512); al-Nawawi stated the sanad is hasan or sahih. (See al-Majmu‘ Syarh al-Muhazzab, 6/317).
 Narrated by al-Bukhari (5404)
 Narrated by Abu Daud (197)
 See ‘Aun al-Ma‘bud, 1/228.
 See Sahih Ibn Khuzaimah, 1/29.
 See al-Minhaj Syarh Sahih Muslim, 4/46.