What is the ruling on some worshippers saying, "Peace be upon us and upon the righteous servants of Allah" immediately after the tasleem at the end of prayer?

It is not prescribed to say: "Peace be upon us and upon the righteous servants of Allah" after concluding the prayer with taslim. This remembrance is only said during the tashahhud and is not among the adhkar prescribed after prayer. Furthermore, adhering to specific acts of worship at times not designated by the Sharia is considered an additional innovation (bid‘ah). The Prophet (peace and blessings be upon him) indicated that after concluding the prayer with taslim, one should say: "I seek forgiveness from Allah" three times, then "O Allah, You are As-Salam, and from You is As-Salam. Blessed be You, O Possessor of Majesty and Honor."

A worshipper's recitation of this phrase is not in accordance with the Sunnah, nor with the Maliki Madhhab; for the taslims mentioned in the Maliki Madhhab (responding to the Imam and to those on the left) are not with this wording.