||Shir ha-Yihud (Hymn of Unity), a lengthy medieval liturgical poem divided into seven parts, one for each day of the week, praising G-d, extolling His uniqueness, and emphasizing the smallness of His creatures. Poetic beauty and sublimity of religious thought have placed the poem among the foremost liturgical compositions. Each line is divided into rhymed couplets, with four beats in each couplet. From the fourth line on, each verse throughout the remainder of the poem contains 16 syllables.
The identity of the author is uncertain and no trace of his name is to be found in any acrostical combination in the poem. Heidenheim (Ha-Piyyutim ve-ha-Paytanim, s.v. R. Judah b. Samuel b. R. Kalonymus, in: Introd. to his Shemini Azeret Mahzor) ascribes its authorship to R. Samuel b. Kalonymus he-Hasid, the father of R. Judah he-Hasid of Regensburg.
Originally, the appropriate portion of the Shir ha-Yihud was recited in many congregations after the conclusion of the daily service. Some congregations only recited it on the Sabbath. The most prevalent contemporary custom is to recite the entire poem at the conclusion of the service on the eve of the Day of Atonement, and to chant the appropriate daily section at the start of the morning service on Rosh Ha-Shanah and the Day of Atonement. Its elimination from the daily and Sabbath services was probably due to the desire not to lengthen the service unduly, though some authorities also quoted the talmudic dictum that no mortal is capable of properly praising the Al-mighty (A. Lewisohn, Mekorei Minhagim (1846), no. 32). "It is as if an earthly king had a million denarii of gold, and someone praised him as possessing silver ones. Would it not be an insult to him?" (Ber. 33b).