Shulchan Arukh, Yoreh De'ah 284/שולחן ערוך·יורה דעה רפ״ד

2It is forbidden to write three words from a verse without scoring lines if it is written in Ashurit script (and there is no difference whether one writes or engraves—whether in wood or in gold )(Beit Yosef in the name of R. Yerucham, Netiv 2); however, in a letter of greeting it is permitted to write even several words from a verse for stylistic beauty. Rema: If one scored a line at the top, then there is no further need (Tur in the name of R. Tam). And there are those who say that one should not write mundane words in Ashurit script—the script used for writing the Torah (ibid. in the name of R. Yerucham).