Matt 5:45 τοις

This represents one of many variations between the Textus Receptus and the Byzantine textform. Many witnesses do not have the article τοις before ουρανοις (ℵ B D L W Δ Σ f1 33 157 565 700 892 1241 pm), and consequently it is not received in the editions of Erasmus, Estienne, Bèze, Mill, Bengel, Wettstein, Matthäi, Griesbach, or even the Byzantine text proponent Scholz, who indicates that manuscripts with τοις are in the minority.
     On the other hand, the Complutensian Polyglot includes it along with a divided but (according to Soden) apparent majority of Greek manuscripts (including K S U Θ Π f13 33 124 205 209 565 661 1006 1342 1424 1506 1582c pm), Clement, Origen (3x), the Apostolic Constitutions, Chrysostom, etc., and it is received in Hodges-Farstad and Robinson-Pierpont, although the latter indicates relative uncertainty by placing the omission in the margin. Matthew certainly makes use of the expression elsewhere both with and without the article:

With the article:
5:16 τον πατερα υμων τον εν τοις ουρανοις
6:1 τω πατρι υμων τω εν τοις ουρανοις
6:9 πατερ ημων ο εν τοις ουρανοις
7:11 ο πατηρ υμων ο εν τοις ουρανοις
16:17 ο πατηρ μου ο εν τοις ουρανοις

Without the article:
12:50 του πατρος μου του εν ουρανοις
18:14 του πατρος υμων του εν ουρανοις
18:19 του πατρος μου του εν ουρανοις

     It is certainly possible that a few scribes could have added the article as apparently happens in a few manuscripts in 10:32, 33 (B f13 892 al), 18:10 (B 892 pc), 22:30 (ℵ B L I f1.13 33 892 1424 al), etc., just as also a few apparently omitted it in 6:1 (ℵ* D Z 0250 f1 33 pc), 16:17 (B), 23:9 (D W Δ Θ f1 pc), etc. Three possible reasons in favor of the addition of τοις include: (1) individual scribes often carelessly omitted especially small and unnecessary words (see note on Matt 1:22 του); (2) it was thought the text would read better without two dental-initial articles on the same line (cf. Griesbach's comment on Matt 1:22 του); (3) it reflects cross-contamination from 2nd-century bilingual Latin-Greek manuscripts, since Latin has no definite article.
     On the other hand, Matthew's style is to omit the article before ουρανοις when the prepositional phrase follows a genitive expression (e.g., 7:21; 12:50; 18:10, 14, 19), and for this reason the large minority of manuscripts (without τοις) is likely correct. Cf. the note on Matt 7:21 ουρανοις for a fuller discussion of the situation.