Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 41
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 41, with English translation by Maimonides (Rambam).
להוסיף קרבן אחר בכל שבת, שנאמר: "וּבְיוֹם הַשַּׁבָּת שְׁנֵי כְּבָשִׂים" (במדבר כח, ט).
To offer an additional sacrifice every Sabbath, as [Numbers 28:9] states: "On the Sabbath day, two lambs...."