הלכה על מנחות 65:9

It is from the roots of the commandment that it be a memory device for a person about faith in God every time that he comes into his home and [when] he leaves [it], and like I wrote about the matter of tefillin. And [it is] like the matter that they, may their memory be blessed, wrote about this commandment (Menachot 33a), "Rabbi Zira said that Rav Matnah said that Shmuel said, 'It is a commandment to place it in the top third.' Rabbah [Rava] said, 'It is a commandment to place it in the handbreadth adjacent to the public domain.' What is the reason? The Rabbis said, 'So that one will be met by the commandment immediately'" (Menachot 33b).