ורבי פנחס הכהן בר רבי חמא אמר הדא הוא דכתיב (איוב לו, יג) וחנפי לב ישימו אף לאחר שהקדוש ברוך הוא מצפה לרשעים שיעשו תשובה ואינם עושין אפילו הם רוצים באחרונה הוא נוטל את לבם שלא יעשו תשובה ומהו וחנפי לב אותן שהם באין ומחנפים בראשונה בלבם הם מביאים עליהם האף באחרונה ומהו (שם שם, שם) לא ישועו כי אסרם אף על פי שהם רוצים לשוב להקדוש ברוך הוא ובאין לעסוק בתפלה אינן יכולים למה כי אסרם שנעל בפניהם כך היה פרעה רוצה לעסוק בתפלה ואמר הקדוש ברוך הוא למשה עד שלא יצא לך והתיצב לפניו:
Pharaoh had finally decided that he would daven to Hashem and really do teshuvah. Hashem therefore told Moshe, “Hashkeim baboker,” go to Pharaoh first thing in the morning -- interrupt Pharoah by appearing before him. Don’t let Pharaoh daven. At some point it is too late to do teshuvah, and so Hashem took the opportunity away from Pharoah.
Why did Moshe have to rush to stop Pharoah from davening? If Hashem did not want Pharoah’s tefilos, he simply could have not listened and responded! And so what if Moshe got to Pharaoh first thing in the morning – there was still the rest of the day?
Two of my children are procrastinators. I once took out of the library for them the book Eat That Frog!: 21 great Ways to Stop Procrastinating and Get More Done inLess Time by Brian Tracy, but they dilly-dallied over reading it, so it did not make much difference. The title of the book has nothing to do with culinary matters – it’s a time management strategy. There are things we know we have to get done, but we really don’t want to do. Imagine you have a big, green frog sitting in front of you that you have to eat. You know it’s going to be a miserable experience, but you have to do it. What would most of us do? We would put off getting it done. We would leave that frog sitting on the plate in front of us and ignore it for as long as possible. We would hope the frog vanishes. The tasks we find unpleasant are just like that big green frog – we push them off and procrastinate. Mark Twain said that if the first thing you do each morning is eat a live frog, you know the rest of the day can’t get much worse. If you tackle the things you are tempted to put off first, the rest of the day goes by much faster and easier.
Davening and doing teshuvah was Pharoah’s frog. Pharoah did not really want to do teshuvah, but he knew he had to. Therefore, Hashem made sure to send Moshe first thing in the morning. Once Pharaoh pushed the frog out the way, the procrastination process would take over and he would not get back to it. Human nature would see to it that there would be no davening later in the day.
Why did Hashem care if Pharoah davened? The Shem m’Shmuel explains that at that time Bnei Yisrael had no zechuyos to speak of. They needed to rely completely on Hashem’s chessed, on getting a free pass without having earned it. Once the highway of pure rachamim is opened between "upstairs" and our world, then it is opened for everyone – even Pharoah can use it. If Bnei Yisrael’tefilos would be listened to without regard to whether they were up to snuff in piety, then the same standard has to apply across the board and Pharaoh's prayers would get through as well. Therefore, Hashem sent Moshe to stop Pharoah from davening.
I would like to suggest a different answer. The reason Moshe had to stop Pharaoh is because tefilah is not some miraculous process – it is something built into nature. Just like if you toss a ball up it falls down because there is a force called gravity that G-d built into the world, so too, if you daven, there are results that come about because that’s the way G-d built the world. It may not be the results you want, it may not happen immediately – but there will be results.