Likutei Sichos vol. 28 page 131 ff.
Para Aduma and a prayer for Moshiach. This Sicha explores the RaMbaM’s prayer for Moshiach in the middle of the laws of Para Aduma and explains that there is a Halacha in this. It is law in Tefilla (hinted and not said outright) that the time to pray for Moshiach is always.