Lectionary 300, designated by siglum ℓ 300 (in the Gregory-Aland numbering) is a Greek manuscript of the New Testament, written on parchment.
[1][4] The manuscript was probably seen in 1761 by the Italian traveller, Vitaliano Donati, when he visited the Saint Catherine's Monastery in Sinai.
His diary, published in 1879, notes: "In questo monastero ritrovai una quantità grandissima di codici membranacei... ve ne sono alcuni che mi sembravano anteriori al settimo secolo, ed in ispecie una Bibbia in membrane bellissime, assai grandi, sottili, e quadre, scritta in carattere rotondo e belissimo; conservano poi in chiesa un Evangelistario greco in caractere d'oro rotondo, che dovrebbe pur essere assai antico".
[6]In this monastery I found a great number of parchment codices ... there are some which seemed to be written before the seventh century, and especially a Bible (made) of beautiful, very large, thin and square parchments, written in round and very beautiful letters; moreover there are also in the church a Greek Evangelistarium in gold and round letters, it should be very old.
The "Bible on beautiful vellum" noted above is probably the Codex Sinaiticus and the gold evangelistarium is likely Lectionary 300.