Some liquid came back up into his mouth when he burped as he was praying; what is the ruling?
This liquid that came out of your stomach when you burped is called reflux; it is similar to vomit but the rulings on it are more lenient. The scholars differed concerning it with regard to two issues, the first of which is whether it is pure and the second is whether it invalidates wudoo’.
The more correct view with regard to these two issues is that it is pure and that it does not invalidate wudoo’.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said in his commentary on al-Kaafi (3/177): There is also no evidence to prove that vomit is impure; it is well known that vomit is very common among people, and it if was impure there would be abundant reasons to narrate reports on how to purify oneself from it. It is also well known that small children vomit in front of their mothers, but there is no report from the Prophet (blessings and peace of Allah be upon him) saying that he instructed that the vomit of a small child be washed away, whereas in the case of urine he instructed that it be washed away and explained the ruling on it. The fact that he did not say anything about vomit indicates that it is not impure. End quote.
Please see fatwa no. 44633 for more information.
If this is proven with regard to vomit, then it is more appropriate that it be proven in the case of reflux.
With regard to it being impure, Ibn Rushd al-Maaliki favoured the view that it is pure and does not invalidate the prayer.
The well-known view is that if vomit or reflux occurs involuntarily and the individual does not swallow any of it, that does not affect his prayer or his fast, but if he swallows it deliberately when he is able to expel it, then there should be no difference of opinion regarding the fact that his fast and prayer are rendered invalid, but if he swallowed it absentmindedly or because he could not help it, then there are two different views.
End quote from at-Taaj wa’l-Ikleel (2/12). See also: al-Bayaan wa’t-Tahseel (1/472); Sharh Mukhtasar Khaleel by al-Kharashi (1/243).
Shaykh Muhammad ibn Ibraaheem (may Allah have mercy on him) said: With regard to reflux the more correct view is that it does not invalidate wudoo’ even if it is impure.
End quote from Fataawa ash-Shaykh Muhammad ibn Ibraaheem (2/74)
In other words, even if we rule it to be impure, it does not invalidate wudoo’.
Based on that, so long as you did not swallow anything of it whilst praying, your prayer is still valid.
And Allah knows best.