Rabbi Isaac ben Jacob or Yitzhak ben Yaakov, nicknamed "ha-Lavan" or "the white" was a 12th-century rabbi of Bohemia. He was a Tosafist and liturgical poet who flourished at Prague in the late 12th century.
He was the brother of the renowned traveler Petachiah of Regensburg. He was among the earliest of the tosafists ("ba'ale tosafot yeshanim"), a contemporary of Rabbi Eleazar of Metz, and a pupil of Rabbenu Tam."Sefer ha-Yashar" §704; Solomon Luria, responsa 29 According to Recanati,Responsa, No. 168 Isaac directed the yeshivah of Ratisbon. He also lived at Worms for a time."Agur," 71b
Isaac is mentioned frequently in the Tosafot,Tosafot to Yevamot 5a, 71a; Ketuvot 38b; Zevachim 73b; and frequently elsewhere and Isaac ben Moses, in his Or Zarua, No. 739, quotes Isaac ben Jacob's commentary on Ketubot, a manuscript of which exists in the Munich Library (No. 317). He is also mentioned in a commentary to the Pentateuch written in the first half of the 13th century.Zunz, "Z. G." p. 80 There is a piyyuṭ signed "Isaac b. Jacob," whom Zunz"Litcraturgesch." p. 313 supposes to be Isaac ben Jacob ha-Lavan.
References
Its bibliography: Azulai, Shem ha-Gedolim, i.; Michael, Heimann Joseph, (1891) Or ha-Ḥayyim, Frankfort-on-the-Main (in Hebrew), p. 507; Zunz, Z. G. pp. 33, 42, 45, 80; Grätz, Gesch. 3d ed., vi. 236; Fuenn, Keneset Yisrael, p. 627.