Wikipedia
Rubāʿī ( rubāʿī) is a poem, or verse of a poem, consisting of four lines. It refers specifically to a Persian quatrain, or its derivative form in English and other languages. The plural form of the word, rubāʿiyāt, often anglicised as rubaiyat, is used for a collection of such quatrains. There are a number of possible rhyme schemes to the rubaiyat form, e.g. AABA, AAAA. In Persian verse, the ruba'i is usually written as a four-line (or two-couplet) poem, with rhymes at the middle and end of each line.
An author described;
" basically Ruba'i belongs to Persia and its metres had been created by a non-Arab poet Abul Hassan Rodeki and that was also brought into practice by non-Arab and Urdu poets. Ruba'i (quatrain) has special metres containing 24 categories (one can say "divisions" or "branches" too). Ruba'i can only be composed in those special metres, not any other normal meter. Ruba'i consists of only four lines, its two lines called 'Sehr' (Stanza). Ruba'i's first and second line must end in rhyme (example-as behold and cold.), third without rhyme, but within 24 special metres, that cannot be changed and fourth line again in the selected rhyme, but that fourth line (misra) contains high, strong and complete and deep meanings, that must be related with above three lines. These should be addressed only one point or subject, not as like ghazals or other forms of the poetry have."
This is an example of a ruba'i from Rūmī's Dīwān-i Shams.''
Anwār-i Ṣalāḥu'd-Dīn bar angēkhta bād Dar dīda u jān-i ʿāshiqān rēkhta bād Har jān ki laṭīf gasht u az luṭf guzasht Bā khāk-i Ṣalāḥu'd-Dīn dar āmēkhta bād "May the splendors of Salahuddin be roused, And poured into the eyes and souls of the lovers. May every soul that has become refined and has surpassed refinement Be mingled with the dust of Salahuddin!"