The measure of the Prophetic sa' and the fact that the standard in measuring is volume, not weight.

The Shaykh : The Prophetic sa' weighs two kilograms and forty grams of good wheat, as we have clarified. But note, everyone, that measuring is determined by volume, not by weight. Is there a difference? Yes, measuring is determined by volume, not by weight. If you measure a sa' with something light, you will find it weighs, for example, one kilogram and five hundred grams. But if you measure a sa' of something heavy, you will find it weighs, perhaps, four kilograms. Take, for example, a small piece of lead weighing ten kilograms and put it in the sa'; it will not fill the sa'. Take something light, like sponge, and put it in the sa'. Fill the sa' with sponge; how many kilograms does it come to? It is very little. Therefore, it is not possible to regulate this by kilograms from every angle, because the standard in measuring is what? Volume, not weight. For this reason, the scholars said that one should be cautious. If one considers weight, one should be cautious. In the case of heavy items, does one increase or decrease? One should be cautious if one wants to be cautious? One increases. Yes, one should be cautious with heavy items, meaning one increases. So, if we find light grain and weigh it, and it reaches two kilograms and forty grams, for example, and then we find heavy grain that reaches two kilograms and forty grams, is it a sa' or less? It is less than a sa'. So we must add to it and say two kilograms and one hundred grams, two kilograms and two hundred grams, two kilograms and fifty grams, and so on. The important thing is that the student of knowledge and the non-student should know that measuring is determined by volume, not by weight. Now, the scholars (may Allah have mercy on them) estimated it using good wheat. "Good" means high quality, not light. They fixed it by weight because weight does not vary. That is, if the scale used for weighing is lost, the weight remains. But if the measure is lost, the measure is lost. Now, what is the ruling on this amount in terms of obligation? The ruling is obligatory (fard). The type is food. The amount is a sa'. The time?