وَنَبِّئهُم أَنَّ الماءَ قِسمَةٌ بَينَهُم ۖ كُلُّ شِربٍ مُحتَضَرٌ

Inform them that the water is to be dispensed between them; every drinking will be attended.

EXEGESIS

The al in al-māʾ (the water) is a definite article to imply that a specific water is intended. It refers to the water of the city which was to be used one day by the camel and one day by the people, as explained in 26:155: This is a she-camel; she shall drink and you shall drink on known days.

Qismah (dispensed) means share, and it is an infinitive in the meaning of the passive participle meaning dispensed and shared.

Shirb (drinking) means a share or allotment of water.

Muhtaḍar (be attended) comes from the root word huḍūr and is the opposite of being absent. The meaning is that the camel would attend the water on her designated day in exclusion of the people and their flocks, and the people would attend the water with their flocks on their designated day in exclusion of the camel. Some commentators say that on that day the she-camel was inspired by God to not attend the water,[1] although it could have been because she did not feel thirsty on that day.

EXPOSITION

Some exegetes indicate that this was the actual test for them.[2] They were not allowed to avail themselves of the water on the day when the camel was to drink. As to why this was ordained, the Quran does not provide any further details. 

Some say that on the day when the she-camel was supposed to drink, it would drink all the water because the quantity of water that was available was very little. However, this seems farfetched in light of 26:146-148 which describes the area as being full of gardens and springs.[3]

Other exegetes mention that the stature of the she-camel was such that it would frighten all the other animals from coming close to the water.[4]

At any rate, even though the people were not allowed to approach the water on the day allocated for the she-camel, according to several narrations the she-camel would allow them to milk her, and miraculously her milk would suffice all the people of Thamūd.[5] It has been attributed to Mujāhid that the phrase every drinking will be attended means every drinking of water and milk, i.e. on the day of your access to the water, you will attend to the water, and on the day that the she-camel accesses the water, you will receive milk from her.[6]

[1] Ibn Ashur, 27/192.
[2] Qaraati, 9/359; Amthal, 17/328.
[3] Amthal, 6/583-84.
[4] Amthal, 17/328.
[5] Kafi, 8/187-189, h. 214; Bihar, 11/386.
[6] Ibn Kathir, 7/444.