What is the ruling on delaying menstruation from its usual time through supplication (du'a), and is there any sin in it?

There is no sin upon you for what you did, as it is permissible to take medication to induce menstruation or to prevent it, so long as no harm results from it. Al-Bahuti stated in "Sharh al-Iqna'" that it is permissible to drink a permissible medicine to stop menstruation or to bring it on, provided that no harm is feared. And abandoning what is permissible is permissible, so there is no sin upon you.