הלכה על מנחות 87:16

And it is practiced in every place and at all times by males, but not by females; because it is a positive commandment determined by time. And nonetheless, if they want to lay tefillin, we do not protest [against them] and there is reward for them; but not like the reward of a man - as the reward of someone who is commanded [something] and does [it] is not similar to the reward of someone who is not commanded [it] and does [it] (Kiddushin 31a). And in Tractate Eruvin 31a in the chapter [entitled] Hamotseh Tefillin, they, may their memory be blessed, said that Michal the daughter of Shaul would lay tefillin and the Sages did not protest [against her]. And there they [also] said the wife of Yonah would go up in pilgrimage and the Sages did not protest [against her]. And one who transgresses this and does not lay tefillin of the arm and of the head has violated eight positive commandments (Menachot 44a); as behold, Scripture commanded about the tefillin of the arm and of the head in four sections.