Is it obligatory to repeat the prayer if a small dry spot is found after finishing the prayer and ablution, knowing that no discharge was felt before or during the prayer?

What you found, a small spot on your clothes, is merely a whisper from Satan, so pay no attention to it and do not repeat your prayer. Doubt is not to be heeded, and obsessive whispers should be ignored. Even if what you found were truly present, your prayer is valid, whether it occurred after the prayer or before it while you were unaware of it. There is nothing upon you.