Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 161
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 161, with English translation by Maimonides (Rambam).
לספור מקצירת העומר תשעה וארבעים יום, שנאמר: "וספרתם לכם, ממוחרת השבת" (ויקרא כג,טו).
To count 49 days from the harvesting of the omer, as [Leviticus 23:15] states: "And you shall count from the day following the day of rest...."