There are certain occasions when we mention the names of our four matriarchs – Sara, Ribka, Rahel and Leah – as part of a prayer. Particularly, in the customary "Mi She’berach" prayer recited on behalf of an ill female patient, we make reference to God’s having blessed our matriarchs, and we list all four by name.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) was asked which sequence should be used when mentioning the final two matriarchs – Rahel and Leah. Although it is common to mention Rahel before Leah, it might, at least at first glance, seem more proper to first mention Leah, as she was the older of the two sisters.
In his work of responsa Torah Lishmah, the Ben Ish Hai writes that in truth it is proper to mention Rahel before Leah. He cites several instances in Torah literature when Rahel is mentioned before Leah. In Masechet Horayot (10), for example, the Torah lists several women in the Tanach who excelled in the area of Seni’ut (modesty), and in this list Rahel appears before Leah. Similarly, toward the end of Megilat Rut, we read that Boaz is given a blessing upon marrying Rut that his new wife should be "like Rahel and like Leah." And in Parashat Vayeseh, the Torah writes that Yaakob called his two wives for a meeting, and Rahel is mentioned before Leah ("Vayikra Le’Rahel U’le’Leah" – Bereshit 31:4).
Therefore, whenever one recites a prayer that mentions the names of our righteous matriarchs, he should ensure to mention Rahel before Leah.
