فَلَ وَرَبِّكَ لَا يُوْمِنُونَ حَتَّىَ يُحَكِّمُوكَ فِيمَا شَجَرَ بَيْنَهُمْ
But no, by your Lord, they can have no faith, until they make you judge in all disputes between them,
Allah swears by His Glorious, Most Honorable Self, that no one shall attain faith until he refers to the Messenger for judgment in all matters. Thereafter, whatever the Messenger commands, is the plain truth that must be submitted to inwardly and outwardly.
Allah said,
ثُمَّ لَا يَجِدُواْ فِي أَنفُسِهِمْ حَرَجًا مِّمَّا قَضَيْتَ وَيُسَلِّمُواْ تَسْلِيمًا
and find in themselves no resistance against your decisions, and accept (them) with full submission.
meaning;they adhere to your judgment, and thus do not feel any hesitation over your decision, and they submit to it inwardly and outwardly. They submit to the Prophet's decision with total submission without any rejection, denial or dispute.
Al-Bukhari recorded that Urwah said,
"Az-Zubayr quarreled with a man about a stream which both of them used for irrigation. Allah's Messenger said to Az-Zubayr,
اسْقِ يَا زُبَيْرُ ثُمَّ أَرْسِلِ الْمَاءَ إِلى جَارِك
O Zubayr! Irrigate (your garden) first, and then let the water flow to your neighbor.
The Ansari became angry and said, `O Allah's Messenger! Is it because he is your cousin?'
On that, the face of Allah's Messenger changed color (because of anger) and said,
اسْقِ يَا زُبَيْرُ ثُمَّ احْبِسِ الْمَاءَ حَتَّى يَرْجِعَ إِلَى الْجَدْرِ ثُمَّ أَرْسِلِ الْمَاءَ إِلى جَارِك
Irrigate (your garden), O Zubayr, and then withhold the water until it reaches the walls (surrounding the palms). Then, release the water to your neighbor.
So, Allah's Messenger gave Az-Zubayr his full right when the Ansari made him angry.
Before that, Allah's Messenger had given a generous judgment, beneficial for Az-Zubayr and the Ansari.
Az-Zubayr said, `I think the following verse was revealed concerning that case,
فَلَ وَرَبِّكَ لَا يُوْمِنُونَ حَتَّىَ يُحَكِّمُوكَ فِيمَا شَجَرَ بَيْنَهُمْ
But no, by your Lord, they can have no faith, until they make you (O Muhammad) judge in all disputes between them."'
Another Reason
In his Tafsir, Al-Hafiz Abu Ishaq Ibrahim bin Abdur-Rahman bin Ibrahim bin Duhaym recorded that Damrah narrated that;
two men took their dispute to the Prophet, and he gave a judgment to the benefit of whoever among them had the right. The person who lost the dispute said, "I do not agree."
The other person asked him, "What do you want then?"
He said, "Let us go to Abu Bakr As-Siddiq."
They went to Abu Bakr and the person who won the dispute said, "We went to the Prophet with our dispute and he issued a decision in my favor."
Abu Bakr said, "Then the decision is that which the Messenger of Allah issued."
The person who lost the dispute still rejected the decision and said, "Let us go to Umar bin Al-Khattab."
When they went to Umar, the person who won the dispute said, "We took our dispute to the Prophet and he decided in my favor, but this man refused to submit to the decision."
Umar bin Al-Khattab asked the second man and he concurred.
Umar went to his house and emerged from it holding aloft his sword. He struck the head of the man who rejected the Prophet's decision with the sword and killed him. Consequently, Allah revealed,
فَلَ وَرَبِّكَ لَا يُوْمِنُونَ
(But no, by your Lord, they can have no faith)