Halakhah sur Daniel 10:12

It is a mitzvah, for every Jewish community, when they are beset with trouble, God forbid, to fast and pray to Hashem, blessed is His Name, (for rescue) from their troubles. But if the time is not appropriate for fasting, for instance, if people are fleeing, they are not permitted to fast, lest their strength will be sapped. However they should resolve to fast a certain number of fast days, when they are saved. This will be counted as if they were now fasting, as we find concerning Daniel, about whom it is written, "He said to me, 'Do not be afraid, Daniel; for from the day you resolved to understand and to fast before your God, your words were heard'" (Daniel 10:12).