Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 113
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 113, with English translation by Maimonides (Rambam).
לעשות פרה אדומה להיות אפרה מוכן, שנאמר: "וְהָיְתָה לַעֲדַת בְּנֵי יִשְׂרָאֵל" (במדבר יט, ט).
To prepare the red heifer so that its ashes will be ready, as [Numbers 19:9] states: "And it will be a keepsake for the congregation of Israel."