(How long have you stayed (here)) meaning, `how long have you slept'
(They said: "We have stayed a day or part of a day.") because they entered the cave at the beginning of the day, and they woke up at the end of the day, which is why they then said,
("...or a part of a day." They said: "Your Lord knows best how long you have stayed...") meaning, `Allah knows best about your situation.' It seems that they were not sure about how long they had slept, and Allah knows best. Then they turned their attention to more pressing matters, like their need for food and drink, so they said:
(So send one of you with this silver coin of yours) They had brought with them some Dirhams (silver coins) from their homes, to buy whatever they might need, and they had given some in charity and kept some, so they said:
(So send one of you with this silver coin of yours to the town,) meaning to their city, which they had left. The definite article indicates that they were referring to a known city.
(and let him find out which is the Azka food.) Azka means "purest", as Allah says elsewhere,
(And had it not been for the grace of Allah and His mercy on you, not one of you would ever have been pure Zaka from sins) 24:21 and
(Indeed whosoever purifies himself Tazakka shall achieve success.) 87:14 From the same root also comes the word Zakah, which makes one's wealth good and purifies it.
(And let him be careful) meaning when he goes out buying food and coming back. They were telling him to conceal himself as much as he could,
(and let no man know of you. For, if they come to know of you, they will stone you) means, `if they find out where you are,'
(they will stone you or turn you back to their religion;) They were referring to the followers of Decianus, who they were afraid might find out where they were, and punish them with all kinds of torture until they made them go back to their former religion, or until they died, for if they agreed to go back to their (old) religion, they would never attain success in this world or the Hereafter. So they said:
(and in that case you will never be successful.)