A tautogram is a text in which all words start with the same letter. Wikipedia

A text in which all words start with the same letter; the difference between a tautogram and alliteration is that tautograms are a written, visual phenomenon, whereas alliterations are a phonetic one. Most cases of alliteration are also tautograms Wikidata

A text, poem or verse in which all words start with the same letter. OmegaWiki

A text in which all words start with the same letter (but not necessarily the same sound). Wiktionary